Every time I logon, with 5 minutes, my cursor freezes and most of the time I get a small window saying I have a unresponsive script. I have to leave the cursor frozen for a minute or two before I can use it. When I play videos, like Youtube, the motion barely shows. It is either frozen or jerky. I use Windows XP and Firefox.
Most likely since you are on XP (Judging your computer would be relatively old) your computer is just affected by it's age.