Hello slepy, welcome to Seven Forums!
If you're going to completely reinstall everything, the best way forward is to start with a complete wipe (secure erase) of the entire HDD to over-write all old code. SSD / HDD : Optimize for Windows Reinstallation
Then using the outline in Step Two #2 of the same tutorial above, create a partition large enough for both Windows 7 and XP, that can be "Resized" using the method outlined in this tutorial below. Clean Install Windows 7 : Ahead of XP
Then install XP first so it will give itself the C: drive letter so that doesn't cause issues; XP will letter the first partition on the HDD as C: and if XP is not on the first partition it will take the next available letter for itself and that can cause issues with installed programs/apps.
Then "shrink/resize" the XP partition so Windows 7 can be installed to the 'created' first partition on the HDD using the method above.
It's an extremely easy and reliable way to accomplish this and is the only way I would do a Windows managed dual boot.
Be sure to post back with any further questions you may have and to keep us informed.