I wanted to clean my computer CPU heatsink and fan itself, because the temperature is not what I wanted. About (50C ~ 70C). I have Intel Core 2 Duo E4300 @1.8 GHz (LGA775).

The heatsink wasn't so scary filled with dust but I wanted to clean it anyway. I didn't know how to get heatsink with fan from the socket. So after 25 minutes I've figured it out. But I didn't know how to get it back on so I spent a lot time getting out the motherboard from the case.

The fan and heatsink... The case and all components are clear of dust. (I'm tired now). Then I put all back just the way it was, well did few things on cable management. But the problem was that I didn't know how to connect front audio connectors. I had Windows XP hibernated. So I started the PC and everything was normal, except CMOS memory was clear. I configured the BIOS just the way it was and while I was doing that I saw about 58C CPU temperature and fan at 1789 RPM. Restarted the computer with new settings applied. But Windows halted with Blue Screen (I forgot what error it was but something with KERNEL).

Restarted the PC and deleted hibernation session and everything was back normal. But couldn't record any sound from front panel microphone. The problem was that I messed ground wire with mic. Again after fixing it I turned computer on. No problems. The fan currently is noisy and temperature was 78C. The temperature before was 55C - 60C at idle. Now it's about 60C. If I do something then temperature raises to 79C. While speaking in skype the temperature was 82C.

Could this problem occur because of the thermal grease (it's old and never replaced)?

Edit
The problem wasn't in thermal paste (because I didn't touch it). The problem was that I installed heatsink wrong. Now instead of regular 60C CPU temperature the CPU is at 48C (cool).