If Windows XP Does Not Show in Windows 7
If your problem is that the XP partition and installation is not showing up in Windows Explorer under "Computer," you may simply need to give your XP installation a drive letter.
To do this, press the Windows key or click the Windows 7 start button/orb and select All Programs → Administrative Tools → Computer Management. In the left-hand pane, click on "Disk Management." After it loads, click your XP partition. If it's not showing a drive letter in parentheses after the drive name, right-click the partition and select "Change Drive Letter and Paths..."
Windows 7's conceit, as well as Vista's, is that it always considers whatever drive it's running from the C: drive. Do not try to alter this. Instead, give your XP partition a higher drive letter than the partition letters already used. For example, if Windows 7 sees a C: and D: drive, re-letter the XP partition to E: or F: (When XP is running, it will show itself running on the C: drive, and the Windows 7 drive will show up as D: or higher.)
After giving the XP partition a drive letter, it should become visible in Windows 7 after a reboot.