Its hard to tell whats goign on in the system
Check Task manager (right click on your toolbar and click "task manager" to see whats' eating your CPU
Definately check out adaware as suggested above... may also want to update your Virus scanner (That you DO have right?!?!?

) and do a full scan.
Media File that doesn't run does certainly scream virus.
do a google search on trendmicro housecall if you don't have one to do a online search
You can also check out
www.grisoft.com for a free virus scanner you can download.