Do you have the Microsoft Office CD?
If you have the CD and the product key, you can simply install it on the new Windows 7 installation. The standard retail license permits you two installations, one on your desktop and one on a portable device, but I doubt Microsoft would object to two installations on the same computer instead. Or, if you replaced your previous operating system with Windows 7, the number of licensed installations does not become a factor at all.
If you have lost the CD but still have your product key, you can download a free trial from Microsoft's website, office.microsoft.com, and apply your product key after installation.
If you have lost your product key and did not register it wit Microsoft, though, you will have to purchase a new copy or use a free alternative, such as OpenOffice.org.
Hope this helps,
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