[sassan]

I agree with Steve as basically his suggestions are exactly what I do too. Including the use of AVG that I find as one the best antiviruses out there and for long, for free.
The last advise is not really needed as you always you can go to:
Start>run>type: msconfig>startup bar (Last option on the right top) and you see all the programs running at start up. I would check any suspicious name (The one that I am not familiar with) on yahoo or google, unselect all not needed (A good time to say good bye to a lot of harmless ones that just occupy the memory for no good reason). As Steve said be careful in doing so (Definitely do it at your own risk) as it is so easy to mess up the entire system here if you are not sure of what you are doing. Backing up the registry before doing any change is a definite step you want to undertake before going any further.