Back up ntldr, ntdetect.com, boot.ini, bootfont.bin under the C drive (the last one is the Chinese version of the boot menu, you can not back it up)
Execute these commands after freshly installing win7
bcdedit /create {ntldr} /d "Windows XP"
bcdedit /set {ntldr} device partition=C:
bcdedit /set {ntldr} path NTLDR
bcdedit /displayorder {ntldr} /addfirst
bcdedit /timeout 12
You can write it into a txt file and then change the suffix name from txt to bat. Run it as an administrator. Finally, copy the backed up file to the root directory of the c drive. After restarting, you can restore the xp boot.