Computer Running Slow/Browser Crashes

My computer will run fine for a while like maybe 10 minutes and after a while it will slow down and then whatever browser I'm using will slow down. Ill be on Internet Explorer and then I will want to use Firefox and then it wont pull up. My computer just slows down for no reason that I am aware of. Virus scans says no items detected so I'm not sure what it could be. Just to let you know, I recently reinstalled windows xp. Here are some other odd things with my computer. CPU is 100% lately with processes Bootup is really slow and so is shutdown. Sometimes when I turn my computer off, ill press the power button and it wont come on. When in standby mode, I cant turn the computer back on.

You say you recently reinstalled Windows XP. Did you also install all the necessary drivers? If you look in Device Manager, are there any yellow or red marks? What happens if you boot to Safe Mode with Networking? Does the computer still slow down or does it run normally?

Looks like the reason why the internet was going slow was because of my ISP. But I checked all the other stuff and it was fine. Browser still crashes more than usual and bootup is still slow. Something like a 30 sec timeout too I noticed.

Do you still have a problem or not? If so, as I asked before,

What happens if you boot to Safe Mode with Networking? Does the computer still slow down or does it run normally?