Laptop really slow and fans rev to max

[atminside]

So I have a pretty old Toshiba laptop A75-S125 which came with a Pentium 4 @ 3.2ghz/HT 533mhz bus, 512mb 266mhz memory, and Radeon IGP 9000, and 40gig 5400 rpm hard drive. It ran on XP. It ran fast and it never lagged. I always clean the insides with compressed air and I even removed the heat sink to re apply new thermal arctic 7 compound. So actually 2 year ago it started to really slow down and the the fans would rev and stay at max. I ran HD tune to check to see if my hard drive was dying but I got 45MB/s average speed which was the same result when I first bought my laptop in 2006. I then formatted and did a clean install of windows. Still it would rev up and was really slow. I even replaced the CPU with a celeron and still the same problem. The funny part is that it would boot up really fast but once i get access to the desktop running any program would be slow as a snail. So the I decided to buy a new 120 gig hard drive and another gig of memory and did fresh install of windows xp. Still the same problem. I checked the heat sinks to make sure it was firmly placed over the CPU and it was fine. I left my laptop in the closet for a while and decided to finally ask for help since i have some free time. I really don't know why it is so slow. Could it have bad capacitors, mosfets, or even a faulty thermal sensor? Could such things be fixed. I really like my laptop and I have a lot free time so I am willing to fix it even if it seems not worth it to other people.
Thanks

 

[C12Friedman]

The symptoms you describe lead me only in one direction - heat/thermal throttling. I'm going to propose here that you reseat your HSF and reapply thermal compound. If there are/were thermal pads on the heatsink, they should also be replaced - thermal compound will not work in their place.

 

[atminside]

I was thinking that maybe the heat sink was not making a proper contact with the cpu. I don't have any pads. Would using a thin sheet of copper plate be a good idea or should i get one of those pads? Also I should mentioned that I lapped the CPU as well.

 

[C12Friedman]

We're going to believe the lapping job isn't an issue here. I'm going to shy away from suggesting a 'shim' of copper since getting good contact there would be near impossible. Thermal pads can be obtained here (higher W/mK is better) http://www.frozencpu.com/cat/l2/g8/c487/list/p1/Thermal-Thermal_Pads_Tape.html
the pads can be compressed up to 50% their thickness but you do want to ensure it is as close as possible (without being too thin) - you don't want tape.

 

[atminside]

Best answer selected by atminside.

 

[atminside]

Thanks I will go and get that pad. Thanks for your help on this matter.