Computer Problem.

Alright, I'll try and explain what's going on with my laptop. It's an emachine, not very fast, not very efficient but it gets the job done. I went on it last night and it randomly froze. I couldn't click on anything and eventually the cursor stopped moving too. I rebooted and it did the same thing and I connected it to it being connected to the internet that seemed to be the problem (though it's done it once without being connected up). After several reboots, I eventually entered into safe mode (which is what I'm in now, but with network capabilites) and it doesn't seem to crash like this. I've done a system restore back three days, to make sure it wasn't a Windows Update but it crashed again today, and I'm starting to suspect it might have something to do with the boot sector, and knowing my luck it's probably a virus.

I tried installing AVG and it told me I have an error just before it finishes it's installation (I've had this ever since I got the computer) so I've been depending on Windows Defender and been very careful about what I do and where I go on the internet.

Can you give me any sort of idea as to what I can do, is there another free anti virus I can use (when doing a scan with Defender nothing out of the ordinary is coming up, but would it with a Boot Sector virus?) and if I do have a boot sector virus, how can I sort this out without having to completely start from scratch?

Or am I completely wrong and there's actually something else wrong with my laptop, as when watching a DVD it just runs fine, without any sort of problem. Oh, and I run Vista.

Thanks, for any help that can be given.