ACK! please help me diagnose my PC its messed up

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hey ive got an issue, every once and a while, My desktop freezes and the clock on the computer stops. i can still run programs if i hit crtl-alt-delete but beyond that the desktop is frozen... the mouse works, but if you put it on the desktop it turns into the black doublesided cropping arrow. ive done virus scans and everything but i do film editing so i really need help... thanks