If the software was preinstalled then you will have some sort of restore disk.
If you dont have a restore disk then you are stuck.
Where did you buy the PC from? I may be that there is a problem with the number of activations that have ocured with this particular copy.
When you are asked to activate, do you get the option to activate by telephone as well as over the net? If so, do so and speak to MS about your problem.