Help with reinstall, possible virus

Thanks for the assistance with this. Here is the sitrep:

2 HDDS. 1 with win Xp regular edition. Tons of data that I don't want to lose. Can set bios to have this as boot up.

Second HDD is my current boot up. It has Windows 7 Upgrade edition.

I have norton security suite and mware malbytes. Both show system squeaky clean.
I have noticed some odd behavior and believe I have a virus despite that. For example, when I type into open office writer, there is sometimes a few seconds delay before the letters show up on screen. Less often this happens in web forums etc. Sometimes this doesn't happen at all. The other issue is firefox frequently gets held up on www.google-analytics while loading video on webpages. Research has told me it may be a virus trying to redirect me.
At any rate, even if I don't have a virus, my system is chugging more than it ever has in the past.

At this point I have backed up my schoolwork onto the winxp drive and win 7 drive can be blown away/reformatted.

If I keep the xp drive plugged in during install of win7 (for the upgrade qualification check), will it be able to spread the virus back over during/right after win 7 install defeating my efforts? I plan on installing mware malbytes and microsoft security first thing.

Any other recommendations? I have the original winXP cd. Should I unplug the winxp drive. Format win 7, install xp, upgrade to win 7 (ouch!) then install malware mbytes +MS security AV and then finally reconnect the winxp drive armed with antivirus. Is there a trick where some of the original win7 upgrade discs didn't ask for validation? Its annoying to think of using tricks and stuff being that I have always been a paying customer...
Thanks for help!!