When you install XP in a dual boot with Windows 7 it takes over the boot until you add Windows 7 back using EasyBCD 2.0.
If you have formatted over XP and only want to restart Windows 7, then make the Repair CD which BFK linked, boot into Recovery Tools to run Startup Repair up to 3 separate times with reboots until Windows 7 starts.
If this fails, make sure Windows 7 partition is marked Active first using DISKPART from the CD command line: Partition - Mark as Active
Then run Startup Repair again up to 3 separate times which should start Windows 7.
If all attempts to start Windows 7 fails, copy out your files if necessary: Copy & Paste - in Windows Recovery Console
If you have a Windows 7 installer you can reinstall using the Product Key on COA sticker on machine. In that case, I would install XP first so that Windows 7 will auto-configure the Dual Boot menu: Dual Boot Installation with Windows 7 and XP
If you don't have or can't locate a Windows 7 installer to clean reinstall, then use your Recovery Disks which should have been made when you got computer, or try running the Recovery Partition using key given on first boot screen or in Manual at Support Downloads webpage for your comptuer. See what options are given to direct install to certain partition, as you might be able to install XP first anyway and then run Recovery.