I would make sure I have a GOOD (not all antiviruses are the same) Antivirus and I would run Malwarebytes (See below for a link) in safe mode.
Good Luck
---------------------------------------------------------------------
These are a few tips to help secure your computer at least somewhat ... .I am not saying non of these programs do not have false-alarms or you'll be 100% protected all the time.. These are just a few tips.
Anti-viruses -
There are many different review sites and many different opinions... I recently saw in a recent (December 2010) PCWorld Magazine a rating of free antiviruses ... I have to say I think its pretty accurate ... Not sure if they rated AVG... but if it was (the ratings were 1 to 5 ... best to worst) than AVG would be #6 (in other-wards, not even on the list)
1) Avast - ( http://www.avast.com/free-antivirus-download ) (Feb 11, 2010 Update to Avast Comment) I always saw potential with Avast, not counting that for the most part (other then perhaps one type of virus ... Avira rates better) Avast rates #1 when it comes to the free scanners and rates higher then MANY shareware scanners (anything not free) ..
After Reinstalling Avast on my older system (5.0.x of Avast), I NO LONGER noticed the significant slowdown I did before ...Compared to Avira, it still lags the system EVER SO SLIGHLTY, but that may be, me being picky
2) Avira - ( http://www.avira.com/en/avira-free-antivirus ) This is what I have on my old laptop and what I had (Trying Avast on my new Toshiba ATM) on my new PC before trying Avast.... I love Avira and even if I am using Avast ... I still recommend Avira 100% ... Detection only slightly behind Avast ... they deducted "points" for the fact they like Avast's Interface better. If I am installing an Antivirus on a system that may not have the largest amount of memory or it may have an older processor .... I choice (just my opinion) to install Avira on it. Even my older system (XP, 768MB Ram, 1.79GHz Processor), has NO slowdown at all with Avira and it's great protection.
Malware/Spyware related -
MalwareBytes ( http://www.malwarebytes.org/ )
For badly infected computers some more options may be Combofix ( http://www.bleepingcomputer.com/combofix/how-to-use-combofix) which I have seen first hand help computers that were badly infected
-Xmetalfanx