And, keep in mind software license requirements may apply to each installation of a particular software. For example, a single-user license for Microsoft Office means Office can be installed on one computer. The exception is that Office can usually be installed on one desktop computer and one laptop computer. So, if you have one edition of the normal single-user license for Microsoft Office, you'll need to decide whether you want to install it with Win XP or Win 7.
If you find having this too restrictive, one option for avoiding the purchase of a second copy of Microsoft Office would be to change your system to one bootable OS, that most likely being Windows 7, and then installing Win XP as a VM, or virtual machine. This would allow you to have both OS running at the same time and switch from one to the other to use Microsoft Office on whichever OS you have it installed.