There are a couple of ways to go here, and neither includes reinstalling the OS, at least not just yet.
The first thing to do if you believe that there is a chance that this is an infection is to open a topic in the Am I Infected forum. The staff there is very well trained, and will be able to best advise you on the proper course of action.
If they determine that the problem isn't related to a virus, then it's time to look at the operating system, and hardware. Faulty RAM can cause a computer to slow down. Damaged files can cause this as well.
If it is a RAM related problem you can download and run Memstest 86. http://www.memtest86.com/download.html
Another thing to look into would be the Event Viewer.
Look in the Event Viewer
and see what errors are showing up at the time that this is happening. Each error will have its own date and time shown.Start> right click on My Computer> Manage> Event Viewer
On the right side of the page you will see the options Applications
, and Systems
, double click on Systems
. There are eight columns on this page, the three that you will want are Type
, and Event
The errors will appear in the Type
column, they are red dots with a white X inside of them.
Warnings will appear as a yellow trinangle with an exclamation mark inside.
When you see an error, double click on it and see what the description is.
Please post the error code/s found in the Event
column and the descriptions back in this topic.