I'm sure ClamWin doesn't have a problem with environment variables, because my XP systems have C:\Users and not Documents and Settings and worked fine.

C:\Users\Public is a more sensible path and equivalent to C:\Documents and Settings\All Users in XP. I don't know why it chose C:\.clamwin.

I'm using Vista Business, I use standard account and installed via the built-in administrator account.

I am not running vista but I am curious. When you installed it as an Administrator did you install it for all users or just the Administrator Account? If you are having trouble using ClamWin in a regular user account you probably selected Administrator only on the install on accident.

No, but a single-user install wouldn't use the root of the drive either.