Has anyone any idea’s

I recently built my first PC I wanted to see how hard it was and if I could build a reasonably cheap but sturdy PC. Below are the Parts listed.
Motherboard: Foxconn Core series
CPU: Intel Celeron 2.5 GHZ Processor
Ram: 512 MB
OS: XP Home Edition Sp2
GPU: ATI Radeon 300X SE 128MB
Now all went fine at first until I decided that I would like to upgrade the Processor to a
Core 2 Duo. 4300 1.8 GHZ Since then I’ve been getting the odd crash the PC just restarts itself back up.
I then get an error report telling me a list of probable causes which are not very specific.
Has anyone any suggestions as to what the problem could be.
Thanks