Computer freezes/slow boot

I'd say I'm okay with working with computers, but this is the first time I've had a problem like this. I'm trying to fix my friend's computer that is taking a very long time to boot and will sometimes freeze. It will pass POST just fine so I'm not sure if its a hardware problem. My other friend has ran a scan on the hard drive so it came out okay. Just did a memtest86+ and it said there was no errors, tried doing memtest again but on probe, but the screen turned black,blue,green, and cyan so not sure on the memory. The graphics card seems fine because displays stuff and once you have the computer boot into a user, he was able to play games. Once you are able to log in, it runs pretty good. I'm not entirely sure if its a software problem either. My friend did a clean install of Windows 7 unless it was installed bad because he did a quick format.

Any ideas on what to do?

Info:
Gateway dx 4300-17
OS Windows 7 x64
 
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