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Hold off on ordering. Something tells me that a new disc won't help. I had a lot of trouble installing the fifth installment myself. Now, the same discs install without any problems. Write to 1C's tech support. But first, scan a copy of your registration form so they can register your game.
As a quick fix: try defragmenting the disc, install the game in selective startup mode, disabling all processes that are not related to Microsoft.
To perform a clean boot of Windows, select "Run" in the Start menu, type msconfig in the Open field, and click OK.
On the "General" tab, select Selective startup.
In the Selective startup section, uncheck the following boxes:
*Process System.ini file
*Process Win.ini file
*Load startup items
On the Services tab, check the "Hide all Microsoft services" box, and then disable all services where the manufacturer is not Microsoft Corporation.
Click OK and select Restart.
(A clean boot may cause some services to become unavailable. Restoring the previous settings may cause the game problems to recur).
Most likely, it's time to reinstall Windows.