Windows.old are just the original "Program Files" folders you had before installing windows 7. If you installed Windows 7 under XP the only way of going back is formating the HDD.
I advice everyone that wants to try this, install it in antoher HDD or a new partition (telling you by experience, i had to reinstall W7 :V)
And this Beta build is actually very complete, i havent finded a single bug and it runs really smooth.
PS:
If you have an ATI graphic card thats not from HD series, DO NOT download the pre-view Windows 7 drivers, they suck.
btw try this cd key:
4HJRK-X6Q28-HWRFY-WDYHJ-K8HDH