getting a black screen of death

I randomly get this "black screen of death" (similar to the Windows blue screen of death).

This started happenning when using Parallels Desktops (note: I used it for a long time w/o any problems), it was up to recently that I strted getting it. At first it was just randomly while using Parallels (running windows), then everytime as soon as I clicked the "launch" button to load windows, to the point where I couldn't access Windows because of it.

I switched to VMWare, everything fine but now I get the same black screen randomly.

The black screen happens mainly when using a VM, now its happening when I have lots of apps running and I use expose (not running a VM).

My solutions are as follow: clearing cache (like in Windows with CCleaner), and disk defrag but since I'm not a Mac 'geek' I want to know a good app to do a similar task as CCleaner (Windows app) and how to defrag my disk.

Also any other recommendations/opinions/solutions/etc from knowledgeable Mac users before opting for a clean OSX install or even an upgrade to Lion.

BTW, I have Snow Leopard...

Thanks..

So by reading some of the other posts, I came across Titanium's Onyx, Maintenance, and Deeper... arte they any good?

Onyx for Lion is highly recommended here.

Thanks.... any other input on this? Maybe someone has experienced something similar?

I have the same problem (black screen of death).

No VM here... Mine does it when I use many applications. But again, it is random... Sometimes I use many applications and it does not do it. I have run the diagnostics and it does not find anything wrong...

I have a mac book pro, 2010 edition, 15 inch.