My PC is just too slow: Can anyone please suggest/advice what should I do?

I have never downloaded any stuff. I have never ever done internet chat.

The computer was running fine till a guest the other day DID INTERNET CHAT!!!! He downloaded MSN Messenger, After that it has been slow.

My PC: SONY VAIO.

I had no problems till that guest DOWLOADED MSN MESSENGER AND DID INTERNET CHAT

PLEASE SOMEONE HELP.