If someone need to use Windows Server 2008 trial for 180 days and make decision for puchasing it.
These are Evaluation Key’s for Windows Server 2008 R2 :

Windows Web Server 2008 R2
= KBV3Q-DJ8W7-VPB64-V88KG-82C49
= 00500-001-0000554-84xxx
= X15-34676

Windows Server 2008 R2 Standard
= 4GGC4-9947F-FWFP3-78P6F-J9HDR
= 00477-001-0000347-84xxx
= X15-34648

Windows Server 2008 R2 Enterprise
= 7PJBC-63K3J-62TTK-XF46D-W3WMD
= 00486-001-0001204-84xxx
= X15-34815

Windows Server 2008 R2 Datacenter
= QX7TD-2CMJR-D7WWY-KVCYC-6D2YT
= 00496-001-0001526-84xxx
= X15-34820

Or may be your guys can go here to get the product codes for server 180 days.
http://technet.microsoft.com/en-us/library/ff793421.aspx