Hello and Welcome to Windows7forums.. 
You can obtain a key for the Windows 7 RC from here:
Windows 7 Release Candidate Customer Preview Program
Just choose which version you have (32-bit or 64-bit) then click "go". In the next screen you'll need to login using your Windows Live ID.. once logged in the next page will give you a product key.. 
You can in fact install without a key but Win 7 will expire after 30 days if no key is entered (meaning you'll get nagging messages telling you that Windows is not Activated).. You can also install without the key and then input the key once your installation is done..
It's up to you, but the easiest route is just to get one and put it in during the installation, then Activate after you've completed the install..
Last edited by Radenight; 05-17-2009 at 04:52 PM.
Operating System Windows 7 Ultimate 64- Bit Edition
Computer Type Custom Build 6 of 6
CPU Type and Speed AMD Phenom II X4 965 (3.4GHZ) 4000MHz HT Quad-Core Socket AM3
Motherboard Chipset Asus Crosshair III Formula AMD 790FX Socket AM3
System Memory Type Corsair TR3X6G1600C8D Dominator - DDR3-1600:PC3 -12800 - 6GB - Non-ECC Unbuffered 240-Pin DIMM
Video Card Type and Speed Sapphire Radeon HD 5870 1GB PCI-Express (2.0)
Computer Monitor Samsung SyncMaster P2350 23" FULL 1080p HD LCD Monitor
Hard Drive (2) WD VelociRaptor 300GB 10000RPM 16MB Buffer: (2) WD Caviar 1.5TB "IntelliPower" 64MB Buffer
Optical Drives (2) LG 22x Super Multi Drives (SATA II) - Both with Lightscribe
Network Adapter Asus Crosshair III Formula Gigabit Ethernet Adapter
Anti-virus Software AVG Free Edition 9.0
Computer Skill Level Self-Taught Expert
Windows Experience Index 7.2