Possible Duplicate:
How do I get rid of malicious spyware, malware, viruses or rootkits from my PC?
Yesterday my gf was browsing one site and clicked one of those tricky and shady ads. After that i noticed her computer was lagging as hell, opened the process administrator and saw "Windows Calculator" using 100% of CPU (no calculator was open, at all, do not recall the exact name of the .exe). I shut it down, computer back to normal and after a couple of minutes, boom, lagging again because the very same process.
Initiated the machine, after few minutes, again, lag. Run AVG antivirus, nothing.
What can this be?