After suffering intermittent "dirty disc" errors, sMoRTy71 decided to buy a replacement DVD drive for his Xbox. He bought one on eBay for less than $30 shipped. The whole process, which involved removing eights screws and disconnecting IDE and power cables, took less than 10 minutes. Read sMoRTy71's personal blog post about his experience replacing the Xbox DVD drive.
CNET Networks - sMoRTy71