What I really want is something I can run now and then for a scan, or to scan individual files. Don't really need something running in the background all the time I don't think.
As far as scanning individual files,
VirusTotal might suit your needs. It's a free, independent scanning service that uses command line versions of all the major AV suites (Kaspersky, Symantec, Sophos, F-Secure, etc). Upload the file you want to analyse and check the break-down of results, based on all the varying AV definitions in use.
I'd never advocate running any MS OS without having a real-time scanning engine on board, however. So it's not wise to rely on VirusTotal implicitly as any kind of substitute.