By Robin Walker [MVP]
After much debugging, I have found that the problem with definition update failures and repetition is because of an incorrect version of the file gcUnCompress.dll in C:\WINDOWS\SYSTEM32\ (for XP) or C:\WINNT\SYSTEM32\ (for 2000 or systems upgraded from 2000).
The incorrect version is of length 130,272 bytes. The correct version is of length 95,448 bytes.
To update your system to install the correct version of the file, do the following:
1. In the System Notification Area, right-click on the MSAS icon and select "Shutdown Microsoft AntiSpyware".
2. In Explorer, navigate to C:\WINDOWS\SYSTEM32\, locate the file "gcUnCompress.dll", right-click on it, and select "Delete".
3. In Control Panel "Add or Remove Programs", select "Microsoft AntiSpyware" and click button "Change".
4. Click "Next".
5. Select "Update Microsoft AntiSpyware", click "Next".
6. Click "Install".
7. When the re-installation finishes, click "Finish".
8. In Explorer, in folder C:\WINDOWS\SYSTEM32\, check that "gcUnCompress.dll" is now 95,448 bytes long, by right-clicking it and selecting "Properties".
9. Launch MSAS, pull down the File menu and select "Check for updates" – this time it really will update your definition files to the latest version.
10. In Explorer, open the folder C:\Program Files\Microsoft AntiSpyware, and satisfy yourself that the modification dates of the three definition files have changed:
You won’t ever have to manually install the definition files again, nor will
the update keep repeating every time you try.