hard drive - Problem with laptop too slow

I just bought a new hard disk for my compaq laptop. I reformatted the hard disk and installed windows 7 32-bit on it.
I have 2Gb of RAM and a core 2 duo 2Ghz processor.
I bought a 320Gb hitachi hard disk, because that's the only thing thats available.
The hard disk that broke is also hitachi.
I've only used that laptop for 2 days after installing the new hard disk. I the laptop for about 6 hours today and its fine. After 1 hour, I decided to turn on the laptop again. That is when it started to get really slow. I haven't installed anything on it before I shut it down. My only suspect is the hard disk is at fault again. My laptop is 3 years old, How am I suppose to determine what the real culprit is

Answer

I have run twice into the issue that accumulation of dust in the air cooling channels/heatsinks degraded cooling so much that the CPU refused to run at the highest clock speeds. So check for some (safe) way to inspect for lint and dust being in the way for airflow and/or heat dissipation and clear it if necessary.

When (trying to) vacuum away the lint from one of said laptops, I managed to just displace a lump of dust in the cooling channel so much that it actually completely blocked the fan from moving at all, resulting in a laptop that kept it's CPU at the slowest speed and refused to run it at any higher rates, regardless of whatever commands I run to explicitly force it to. (At times, IT-equipment demonstrates fascinating ways of protecting themselves from damage). Needless to say, a laptop that has a max CPU of 2.2GHz feels quite sluggish when it refuses to run faster than it's slowest 800MHz.

If you use some sort of vacuum cleaner, be careful that you don't allow the fan to spin freely when cleaning (it might get damaged I've read). Just block it using some toothpick or so. (Of course, be always careful when you do vacuum clean computers, especially with covers removed. Vacuum cleaners love to do surprise attacks on delicate circuit boards they come nearby, not to mention their apetite for little keys.... sluuurp).