(Install Issue) Can't Create/Locate new partition

I am running vista 64bit ultimate, i have two HDD a 640gb for games and media.. and a 320 for my OS, both West. Dig.. I partitioned 20 gb on the 320 using the vista partition tool, and when i try to install windows 7 on this partition... i get the cant create/locate new partition error... so i got fed up and was just going to install over my OS ,,, but that partition is giving me the same issue... whats the deal?

If the 20 gig partition is unallocated, you might have to format it first. Does it show up as a partition in Vista under my computer?

I formated it multiple time, however my issue was resolved. I do appreciate the help, I did three things, reformatted it to 50Gbs, Unchecked quick format, and reset my bios so that the 320gb HDD was first in boot order.... which appearently was reset because it used to be.. so now im currently in Win7 not sure what of those three fixed it but i suppose it doesnt matter. Getting ready to test Left 4 Dead to check gaming Performance