Maintenance and Service

I have a Precision 650 Workstation that needed a motherboard and powerpack. I sent it out to a Dell recommended service center and while the machine runs all of the applications it is VERY SLOW to connect to the Internet. The device manager indicates the network adapter is working correctly. Anti-virus and Spyware software has not found anything. The machine has been defraged and unnecessary files have been deleted. I tried to resolve the problem by reinstalling the OS but to my surprise the machine reads the resintallation disk, goes through the process but does not install a fresh copy of the OS or reformat the hard drive.

Does anyone have any idea how to correct this problem? And does Dell have its own repair service other than independent repair technicians they sub-contract to?