It sounds to me like your doing some very heavy duty music tasks on a note so heavyweight computer set-up.

The reasons for it happening? Who knows? I recently had a similar problem where Cubase kept crashing whenever I opened a song which contained virtual synths. What would happen is that it would play the first split second of the song, then the audio would drop out and the thing would crash.

I'm not entirely sure what the problem is, but I can offer some solutions. The way I see it, you've got two choices- you can either upgrade to a mega computer with loads of RAM and a 1000+MHz Processor OR you can change your Windows Operating system to Windows XP. Strangely enough, the second option is not only cheaper but it also promisses the best results.