Stalling / Crashing (Vista)

Not sure what this qualifies as, but here it is.

Every so often when trying to boot up, I have to keep trying to get it to boot by turning the computer off, and retrying. This usually has to happen close to 10 times at times, sometimes it works the first time. When it does stall, it does so at the little Microsoft loading bar.

As well, it tends to freeze up completely if I try and launch a game, more recently, Borderlands. Sometimes it freezes up at the splash screen, sometimes it crashes midway through, and occasionally it'll crash during gameplay and yeild the blue screen with a different error every time.

I'm not sure what is doing this. The only thing installed recently was a new hard drive, as apparently my old one had burned out, as well as a better power supply as the old one finally went out after 6 years.

Any help would be much appreciated.