So I'm finally upgrading and going dual core and getting a 8800GT & 2gb of fast memory. However over the the last 5 years I've been using the same hard drive for my operating system. A basic 40gb samsung one.

I was planning on formatting it and installing a fresh copy of windows on it when I build the new PC, and am wondering if I should buy a brand new hard drive to install it on? Would my old hard drive make my pc slower at all? I don't wanna go buying good stuff then having it slow windows down.

well if you are using 40 GB HD I assume you are using an IDE interface dirive IFf you can afforad a new hard drive then switch to SATA and faster hard drive like raptor then you can get a performance improvement... but before doing all that make sure your mother board supports hard drives

You definitely need a new hdd, the life expectancy is at best 5 years. Agree you should get a SATA. Given you're using a 40GB hdd, you don't have a lot of data so an 80GB SATA Seagate would work, which would be under $50. There is nothing wrong with the Raptor but if you've been using a 40GB hdd for 5 years, you're not likely doing anything that would justify the cost of a Raptor.

The slowest component of any pc is the HD, meaning that usually all the other parts has to wait for the HD and not the other way around.

Upgrading to the WD AAKS will bring significant performance boost to your pc in applications and loading times for games.