Custom built computer running slow

A few weeks ago, I built a computer. When I was first about to install Windows XP it shutdown because the processor overheated. It was at 100 C. I checked in the case and the heatsink wasn't secured on all the way like I thought. So I secured it down again and it the processor temperature went down to 40 C. OK, good. A week or 2 later when I installed Windows XP, it took way longer than it should have compared to how long the estimated time said. I mean it took like 2 to 10 minutes for the estimated time left to go down by 1 minute. It seems like it installed fine but, it runs very slow. When it boots up and shows the Windows XP logo and progress bar, the moving bars look like they freeze for a half second here and there because of slowness. When I log on, it's just as slow. Navigating the start menu, running anything, everything is slow. There aren't any message boxes about corrupt files, so that's not my first thought. I have a feeling it might be the processor. Maybe defective or maybe damaged by overheating. Even though the pentium 4 is supposed to prevent damage by slowing down if it overheats, maybe without the heatsink down enough, it still heated enough to get damaged. But first I need to get other people's input to see if there's anything else I'm not thinking of.

Here's the computer's specs:
Intel D915PGN Motherboard
Intel Pentium 4 Processor 3GHz, 2Mb cache, 800MHz bus, supports hyperthreading and 64-bit computing
Micron DDR SDRAM 400MHz, 2GB (4 512MB sticks)
MSI ATI Radeon Video Card RX300HM (256MB DDR2 RAM)
Creative Labs Soundblaster Live Sound Card
Hitachi Deskstar 160GB 7200 RPM SATA Hard Drive
LiteOn 20X DVD+RW SATA Drive
Antec 350Watt Power Supply

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