stuart,
please try the following:
1. kill
all running programs
2. kill
all avast components (from the Processes tab of Task Manager - namely ashsimpl.exe, ashDisp.exe, ashServ.exe, aswUpdSv.exe, ashMaiSv.exe, if running)
3. place the file
http://cat.asw.cz/~vlk/aswScan.dll to your avast folder (it should be possible [should not trigger a sharing violation error] since you did what I said in step 1. and 2.)
4. try it again. It should help.
Please be prepared that the avast auto-updater will soon overwrite the file with the original version (and will possibly want you to reboot), but that's OK - this is just to test if the update worked...
Thanks
Vlk