@Inaris - You don't have to register (see below). You do have to activate.
Is it asking you to register or to activate?
When Windows first boots up, after being installed, you're asked to add at least one account and then register. You can get around this by simply rebooting.
Activating on the other hand is something you have to do before you can use Windows, and the only way around that is to either activate or use a crack to bypass activation, which I would only recommend you pursue if you have a legit copy of Windows. Otherwise, you may be doing something bad.
AMD Athlon64 3200+ S939
1024MB Dual Channel DDR400 Corsair XMS with LED activity indicators
256MB GeForce 6600 PCI-E by Chaintech
Aspire X-Navigator case with 500W PSU, 6x80mm fans, 1x120mm fan
Gigabyte nForce4/PCI-E S939 mobo
80GB 7200MB 8MB WD IDE HDD
16X DVD±R Dual Layer burner
Memory card read/writer
...and a Led Zeppelin sticker!