1. Are you suggesting you created a system restore point in XP then clean installed Win7? If so, you are out of luck.
2. If it was a clean install to a separate partition as a multi-boot, download VistaBootPro from here click here
Run it in Win 7, go to Manage OS Entries > Highlight your Win7 installation > Click X > Confirm the deletion. Restart the computer. The partition that occupied Win7 can now be formatted via Disk Management.