my macbok keeps freezing, possible problem is?

I can already see how this might go – but it’s not working so I have few choices.

I bought a white macbook (3.1) from a friend which was kitted with 1GB ram and a 80GB HDD. What I wanted was a windows based laptop, but she needed the money blah blah.
I upgraded the ram to 4Gb and HDD to 500GB. Both were bought from sellers on ebay, both stated they were macbook friendly.

The mac came with no install discs, they were ‘lost’, but having owned a macbook in the past and the optical drive breaking, I backed up a copy of OS X onto an external drive.
Trying to install it gave me the ‘You cannot install OS X on this computer’ error message and leave me up the creek without a paddle.

Eventually, I found a method of installing windows straight onto the macbook bypassing the need for bootcamp or even OSX, so I did. And everything went through fine.
I had installed a copy of Win 7, and it worked like a dream, but would occasionally freeze.
The internet would be running and all of a sudden firefox would just stop responding, everything else was still active, but would take forever to respond – for example, in that freezing state, I’d press start and go into note pad. 5 minutes later, note pad would open, but the start bar would still be up.

After a couple of days I decided to downgrade to Vista HP as I figured although the RC win 7 works fine on my desktop, on a macbook there may still be a few issues.
Vista was cool for a minute, then the same as above. I was trying to listen to a 10 minute radio interview that took the best part of an hour.

Given when using bootcamp, you are creating a whole new partition and installing windows, I cant see why having OSX installed on the machine would make a difference, but software wise, that was the only difference between this macbook and my previous one, and the previous one worked without a single problem.

So now I’m wondering if there is an issue with the RAM or HDD and I have no idea how to test what it may be.
Last night I was trying to watch a DVD and the whole thing froze up after 10 minutes. The fan eventually shut up, and the laptop just froze. If I leave it long enough, it will unfreeze – as with the interview, but my patience is already running out.
Any ideas?

No fanboy talk please.