Version 6 could be registered internally (Maintenance folder of avast interface).
Please, upload (attach) the avast log: C:\Program Files\Alwil Software\Avast5\Setup\setup.log
(or the shorter C:\Users\All Users\Alwil Software\Avast5\log\Setup.log).
If the file is too big for the forum, post the last 400-500 lines of it.
11.04.2011 20:09:40.000 1302574180 (null) Started: 11.04.2011, 20:09:40
11.04.2011 20:09:40.000 1302574180 (null) Running setup_ais-3e8 (1000)
11.04.2011 20:09:40.000 1302574180 (null) Operating system: Windows 7 ver 6.1, build 7600, sp 0.0 x64
11.04.2011 20:09:40.000 1302574180 (null) Memory: 27% load. Phys:3054500/4192824K free, Page:4194303/4194303K free, Virt:2046260/2097024K free
11.04.2011 20:09:40.000 1302574180 (null) Computer WinName: WALLY-PC
11.04.2011 20:09:40.000 1302574180 (null) Windows Net User: SYSTEM
11.04.2011 20:09:40.000 1302574180 (null) Cmdline: /downloadpkgs /noreboot /updatevps /verysilent /session "0" /limitcpu
11.04.2011 20:09:40.000 1302574180 (null) DldSrc set to inet
11.04.2011 20:09:40.000 1302574180 (null) Operation set to INST_OP_UPDATE_GET_PACKAGES
11.04.2011 20:09:40.000 1302574180 (null) Old version: 3e8 (1000)
11.04.2011 20:09:40.000 1302574180 (null) Deleted registry: Software\AVAST Software\Avast\UpdateReady
11.04.2011 20:09:40.000 1302574180 (null) Using temp: C:\Windows\TEMP\_asw_aisI.tm~a02296 (80995M free)
11.04.2011 20:09:40.000 1302574180 (null) SGW32AIS::CheckIfInstalled set m_bAlreadyInstalled to 1
11.04.2011 20:09:40.000 1302574180 (null) SYNCER: Agent=Syncer/5.00 (ais-1000;p)
11.04.2011 20:09:41.000 1302574181 (null) Computer DnsName: Wally-PC
11.04.2011 20:09:41.000 1302574181 (null) Computer Ip Addr: 192.168.1.64
11.04.2011 20:09:41.000 1302574181 (null) Installed in: C:\Program Files\Alwil Software\Avast5 (80995M free)
11.04.2011 20:09:41.000 1302574181 (null) SYNCER: Type: use IE settings
11.04.2011 20:09:41.000 1302574181 (null) SYNCER: Auth: another authentication, use WinInet
11.04.2011 20:09:41.000 1302574181 (null) Part prg_ais-3e8 is installed
11.04.2011 20:09:41.000 1302574181 (null) Part vps_win32-11041102 is installed
11.04.2011 20:09:41.000 1302574181 (null) Part setup_ais-3e8 is installed
11.04.2011 20:09:41.000 1302574181 (null) Part jrog-a7 is installed
11.04.2011 20:09:41.000 1302574181 (null) Part jrog2-1bd is installed
11.04.2011 20:09:41.000 1302574181 (null) LoadState: Edition=2
11.04.2011 20:09:41.000 1302574181 (null) Old version: 3e8 (1000)
11.04.2011 20:09:41.000 1302574181 (null) skipped CPackageEngine_File::SetExistingFilesBitmap
11.04.2011 20:09:41.000 1302574181 (null) GUID: 5e50a93d-0c68-40b1-ae82-4072b2082a88
11.04.2011 20:09:41.000 1302574181 (null) Server definition(s) loaded for 'main': 299 (maintenance:0)
11.04.2011 20:09:41.000 1302574181 (null) SelectCurrent: selected server 'Download332 AVAST5 Server' from 'main'
11.04.2011 20:09:41.000 1302574181 (null) SYNCER: Type: use IE settings
11.04.2011 20:09:41.000 1302574181 (null) SYNCER: Auth: another authentication, use WinInet
11.04.2011 20:09:41.000 1302574181 (null) SYNCER: Agent=Syncer/5.00 (ais-1000;p)
11.04.2011 20:09:41.000 1302574181 (null) Used server:
http://download332.avast.com/iavs5x11.04.2011 20:09:41.000 1302574181 (null) GetFileWithRetry: servers.def.vpx downloaded .
11.04.2011 20:09:41.000 1302574181 (null) servers.def.vpx not changed, 1299350494
11.04.2011 20:09:41.000 1302574181 (null) Server definition(s) loaded for 'main': 299 (maintenance:0)
11.04.2011 20:09:41.000 1302574181 (null) SelectCurrent: selected server 'Download696 AVAST5 Server' from 'main'
11.04.2011 20:09:41.000 1302574181 (null) SYNCER: Type: use IE settings
11.04.2011 20:09:41.000 1302574181 (null) SYNCER: Auth: another authentication, use WinInet
11.04.2011 20:09:42.000 1302574182 (null) Used server:
http://download696.avast.com/iavs5x11.04.2011 20:09:42.000 1302574182 (null) GetFileWithRetry: prod-ais.vpx downloaded .
11.04.2011 20:09:42.000 1302574182 (null) prod-ais.vpx not changed, 1302552493
11.04.2011 20:09:42.000 1302574182 (null) LoadProductVpu: C:\Program Files\Alwil Software\Avast5\Setup\prod-ais.vpx
11.04.2011 20:09:42.000 1302574182 (null) LoadPartInfo: jrog = jrog-a7 returned 00000000
11.04.2011 20:09:42.000 1302574182 (null) LoadPartInfo: jrog2 = jrog2-1bd returned 00000000
11.04.2011 20:09:42.000 1302574182 (null) LoadPartInfo: program = prg_ais-3e8 returned 00000000
11.04.2011 20:09:42.000 1302574182 (null) LoadPartInfo: setup = setup_ais-3e8 returned 00000000
11.04.2011 20:09:42.000 1302574182 (null) LoadPartInfo: vps = vps_win32-11041102 returned 00000000
11.04.2011 20:09:42.000 1302574182 (null) LoadProductVpu: C:\Program Files\Alwil Software\Avast5\Setup\prod-ais.vpx ended with 00000000
11.04.2011 20:09:42.000 1302574182 (null) Customer 999999
11.04.2011 20:10:01.000 1302574201 (null) submit has nothing to send
11.04.2011 20:10:01.000 1302574201 (null) Submit: files 0, bytes 0, time 0 ms
11.04.2011 20:10:01.000 1302574201 (null) Submit success: files 0, bytes 0, time 0 ms
11.04.2011 20:10:01.000 1302574201 (null) Transferred: files 2, bytes 0, time 780 ms
11.04.2011 20:10:01.000 1302574201 (null) Retries: total 0, files 0, servers 2
11.04.2011 20:10:01.000 1302574201 (null) NeedReboot=false
11.04.2011 20:10:01.000 1302574201 (null) Return code: 0x20000001 [Nothing done]
11.04.2011 20:10:01.000 1302574201 (null) Stopped: 11.04.2011, 20:10:01