If you are trying to install Windows XP or Vista you won’t be able to validate your install until Tuesday so says Microsoft:

We contacted our sources at Microsoft, who told us off the record that the company is aware of a major WGA server outage affecting users across the globe. The Windows Genuine Advantage support forum has exploded with complaints, as a result, and Phil Liu, WGA Project Manager, says that he won’t sleep until the problem is fixed. Windows Vista and XP are affected, 32- and 64-bit versions.

Via ars technica