I had Excel 2007 on Windows XP and now just upgraded to Windows 7. Excel 2007 (also tried the beta of Excel 2010) does not work well. The PASTE function losts it PASTE FORMULA and other paste special options. Sometime, it shows up but most of the time it's not there. CUT/INSERT cut cells are also gone. What happens?
Classic case of "if it ain't broke don't fix it." You've proverbially screwed yourself by upgrading to Windows 7 because there's no easy way to go back to Windows XP, but here is the one and only way (without losing everything):
The options are still there in office 2010, just moved (don't ask me where because I avoid spreadsheeting whenever possible lol). I think the specific special paste options were replaced by key commands... but again, I am not sure on this.
Office 2007 works perfectly fine on win7 (I used it back when I was on XP, and carried it over to win7 and saw no differences). It is the exact same software no matter the OS it runs on, and it behaves the same way on both XP and win7, so perhaps try reinstalling it (or maybe you had Office XP/2002 before?). Sometimes (though not often) functionality can be lost when upgrading the OS on a computer. Try win7 from a fresh/full install and see if that works better for you.
To go back to XP you will have to reinstall the system.
1) backup all of your important files to CD/DVD/cloud/flash drive
2) Insert and boot from your XP CD or boot from the recovery partition if your computer did not come with restore discs
3) when choosing where to install the OS be sure to first delete all partitions, make a single large partition, and then format it before installing XP (these options are found when booting from the CD, I am not sure how it works from a restore partition)
4) reinstall all drivers/software/updates, and copy all of your files back onto your computer
If your PC did not come with restore discs there is a chance that you deleted your restore partition when you upgraded to win7. To get around this you can reinstall from any XP CD (borrow one from a friend) and then use the license that is on your XP sticker attached to the case. The other option is to order restore discs from the manufacturer which typically costs $30-50, but if the system is too old they will refuse to sell you the discs and try to get you to buy a new system. If neither of those options work then you will have to buy a copy of XP.