It could also be the heatsink has dried up. this is a common issue, do you hear any specific sounds when you try and put the machine on? If you want, you can get the mobo repaired too (if possible), if you are in mumbai, you can go to esskay in lamington road and give it to them. If its the heatsink paste, you can get Arctic Silver 5. Its an amazing product. Just make sure you read the online application manual. A single drop (size of uncooked grain of rice) is generally good enough for a p4 system. Not to mention it brings down the core temperature upto 8-10 degrees when you clean the fan + apply this. The cooler your machine, the faster/better it performs.
Alternatively if you are considering a new system, amd's are okay, but if you want to be future proof, the core 2 duo would be a better bet. AMD64 gives you the 64 architecture, but its still officially an emulated architecture.Unless you went for the opterons and stuff. The core2 would give you the benefits of SMP (if enabled in your kernel).
I normally prefer AMD products to Intel (from the early 700mhz duron, and more with the t-birds and even more since the athlons), have always favoured the underdogs, but bottom line is still performance. But this would however mean a bit more expenses now, as compared to performance you gain in the long run.
So my preferance Core 2 Do + P5W DH Deluxe [ http://in.asus.com/products.aspx?l1=3&l2=11&l3=248&model=1198&am... ] P5VD2-X -http://in.asus.com/products4.aspx?l1=3&l2=11&l3=346&model=1362&a... Good luck
Regards, Satish