Yep, could be your drive, but the fact is, Vista uses the lowest scoring component in your system as your "score".... so just open up the Windows Experience Index and see what component is causing your score to be so low....
For reference, my primary Hard Disk is a 5 year old 80g Barracuda and it scores a 5.2....
And really, those scores don't mean shit anyhow.....
You need to look at the analysis and see which component is scoring the lowest. The WEI is always equal to your lowest score, so if you have 5.9 memory, cpu, and graphics but a 4.0 disk, then your total score will be 4.0
Most people with a single drive system will always be limited by their hard drive. AFAIK only a RAID array will reach 5.9.