PC wont boot

Hello there!

I was having problems with my computer which led to it freezing quite often so i decided i would change the heatsink and fan on my chip. Now as i know very little about computers, i realise in hindsight that this was probably not a particularly good idea! Anyway as it turned out i had bought a new heat sink and fan that did not have the same connection as the one originally on the motherboard so i put the original back on and closed up the case. From that moment my pc would not power up apart from a 3 second spurt. I removed the case again and checked all the connections to see if i had loosened anything up, tryed again and still nothing. Following that i removed the power supply and tried it on an old computer and it worked so i assummed that it is still okay. Now as i have no technical knowledge of computers i am unsure of what i can do to rectify this situation. If anyone has any useful ideas i would be extremely grateful!

Thanks in advance guys,

Brumnick

Oh yeah....
2 gig AMD Athlon XP
1024 meg memory
Windows XP Pro Service Pack 2