Avast crashes trying to update virus databases

[zlo]

this has started only recently. couple of days ago.
i haven't touched AVAST in any way, but now every time it tries to update its virus databases (both automatically and manually), the app crashes (windows VISTA) with the following on-screen dialog presented by the OS:

Problem signature:
  Problem Event Name:   APPCRASH
  Application Name:   avast.setup
  Application Version:   4.8.0.0
  Application Timestamp:   482d2764
  Fault Module Name:   avast.setup
  Fault Module Version:   4.8.0.0
  Fault Module Timestamp:   482d2764
  Exception Code:   c0000005
  Exception Offset:   0005021b
  OS Version:   6.0.6001.2.1.0.256.1
  Locale ID:   2057
  Additional Information 1:   7136
  Additional Information 2:   01ccbfe8524fff66028a6b5d6aa6614a
  Additional Information 3:   1457
  Additional Information 4:   0b09848753c439faaeb53538b700dd6a

[rassel]

Try to repair avast by going Add and Remove Programs>Avast>Repair

If the repair does not work, then Uninstall avast and reinstall.

[Forejt]

Edit file setup.ini in avast4\setup folder and add the following lines:
[BufferLog]
Enable=1

Save the ini file, then start update. When it crashes, send me file SetupDbg.log from avast4\setup folder.
After that, remove those 2 lines from setup.ini.