How to Remove OS in dual boot?

I recently clean installed Windows 7 x64 Ultimate over my previous OS Windows XP, so my question is how do i remove the Windows XP OS and keep the Windows 7? i both installed XP and Win7 on Partition C

hmm i looked at the guide but i dont know what is the active partition between the two?
Here's whats on my Disk Management
( C: ) Simple Basic NTFS Healthy(System,Boot,Page File,Active,Crash Dump,Primary Partition)
( D: ) Simple Basic NTFS Healthy(Logical Drive)

here sir

oh didn't see that o.O thanks, i think i might not remove windows xp anymore too complicated steps

It appears you didn't boot the Win7 DVD to install and instead ran it from XP, which locked out the C letter so you don't have Win7 on C as it always is when correctly installed from boot. So I would consider reinstalling it correctly from boot while deleting all partitions during a Clean Install Windows 7.

If you want to keep what you have then since D is a Logical partition which cannot have the System boot files written to it, you must first convert it to Logical. Download free Partition Wizard Boot CD, burn to CD using Windows Image Burner, then boot CD, rightclick D, select Modify>Set to Primary, click OK.

Next rightclick D to Modify>Set to Active, OK.

Then click on HD to highlight it, from Disk tab select Rebuild MBR, OK, Apply all steps.

If Win7 doesn't start at reboot, boot into Win7 DVD or REpair CD to run Startup Repair - Run up to 3 Separate Times until Win7 starts and holds the System flag.

You now know Win7 can boot from its own partition, so you can boot the PW CD, rightclick C to delete, click OK, then rightclick D to Resize, slide left border all the way to left, OK, APply both steps. Partition Wizard Move/Resize Partition Video Help.