MacBook aluminium Random freezing problem

Hello, I bought my friend's Macbook aluminium (2008) it orginally had a 250gb HD installed but he wanted to keep it. I therefore installed my 80gb HD from my old white macbook - however at times when i start it up the macbook freezes upon the login screen where i have to manually power it off (this also freezes happens once i close the lid).

I have turned off the safe sleep mode via command- Is it a problem that i have installed a much smaller HD than what it orignally had? Also why does the computer freeze when i close the lid even though i have set hibernate to 0?

It is running of 2gb of RAM but even running safari at times takes a while to load. Apparently the macbook had no problems at all before the HD swap.

Any adivce would be much appreciated,

Thanks Michael