Windows? (mac here, so no experience with checking for viruses, etc.)

I use Pro, Free, CVS versions and others, and I can't say one slows my machine down more than another. If a file looks good with many sources but goes immediately to need more sources, I figure it's spam and look elsewhere.

Try downloading a known legitimate file, like an episode from + MagnetMix.com +: The Source for Good Content

That should work well. Also, if a file seems suspect, right-click it and see if a Bitzi lookup has any info on it.

Cheers