Reinstalled Vista Business 32-Bit SP2, now experience random freezing

Hi everyone,

I'm hoping for help with my laptop, which has started freezing at seemingly random intervals.

I recently did a clean install of Vista because although it was working, it was rather slow and Firefox kept stopping responding every couple of minutes whenever I was browsing the internet.

I've previously done a clean install of Vista on the same machine without problem so felt confident to do it again.

My laptop originally shipped with Windows XP and I got sent a Vista Upgrade disk as I'd bought it within a few months of Vista being rolled out.

Anyway, a week or so ago, I did the clean install of Vista and downloaded/installed all the other programs I use.

But since then, my laptop has started completely freezing - seemingly randomly. Sometimes it runs fine for hours on end before freezing, but other times it doesn't last as long. When it freezes up, everything stops. The hard drive light goes out, the mouse pointer won't move using either the track pad or the USB mouse, the CAPS lock button on the keyboard doesn't turn the CAPS Lock light on, the Avast icon in the system tray stops mid-animation and basically I can't do anything apart from switch the machine off completely by holding in the power button until it powers down.

I've tried using CCleaner to get rid of junk and clean the registry, I've run Malwarebytes (which found nothing), I've used Defraggler and got it down to 1% defragmented, I've checked the hard drive using chkdisk and found instructions to use the System File Checker, which seemed to fix a bunch of things but then, after a while, the computer still froze up.

I downloaded HD Tune Pro and checked the hard drive but it reported that there were no damaged blocks.

I've now used CCleaner to stop some of the startup programs but that hasn't fixed anything either.

Can anyone help me fix this annoying problem? I'm not sure what to try next and am getting really frustrated with it.