On a 256 Mb, XP pro Dell Optiplex, it took about 5-10 minutes to get the prompt to restart. After the restart, though, Sophos is still updating. I'm not exactly sure when that process is over, but when it is, mousing over the shield in the system tray will tell you when the last update was.
After the restart, however, I got this error:
LDIScn32.exe - Unable to locate component
This application has failed to start because loc32vc0.dll was not found. Reinstalling the application may fix this problem.
That issue was resolved by following the link in RussellHltn's post above:
RussellHltn wrote:No, run the Sophos first. Link