I have two hard disk drives, each of 1TB size.
How can I merge them and make them one drive in Windows Server 2008?
After doing that, how can I reinstall Windows on those hard drives, keeping the data?
You are looking for RAID0, also known as striping. Check out this web page for more information.
As far as keeping data on both drives? You'll need to transfer it off the drives and recopy it over. I HIGHLY recommend having a backup and restoring it with that method. Although RAID0 is faster then individual disks, you are DOUBLING your risk for data loss on BOTH drives. Make sure all critical data is backed up!!!