Skotman:
Though you can extend partitions over multiple disks, you must also realise one possible future problem with it: if either of the drives fails, you lose data of the entire partition.
Alanterrill:
One thing you might want to do is re-install BackupExec on your D-drive. That way you get some free space on the c-drive. Don't forget to modify your backup as well to to include the part on D where you install BackupExec (if you want to).
Wether or not you can extend partitions outside your raid 5 set, depends on the 'pimpness' of your raid controller. If it supports it, you might have basic disks in a raid 5 configuration (yes, this is possible with hardware raid controller), in that case you need to upgrade them to dynamic discs.
Also, it might be possible for you to add another disk to your raid 5 set, if you are considering buying another disk as an option.
Good luck!