Microsoft Office is completely separate to your operating system (Windows XP or Vista). If you've reformatted your hard drive, you'll have wiped all of the software from it. Reinstalling your operating system won't reinstall anything else. You need to install each program separately, including MS Office.
If your PC didn't come with MS Office on disk, you can't now reinstall it without paying for a new copy (which will cost you around �80).
The alternative is to download Open Office. It's at least as good as MS Office and it's (almost) fully compatible with it. i.e. if someone sends you a document created in MS Office, you'll be able to read it and modify it. Similarly, if you create a document in Open Office, you can send it to someone using MS Office without any problems.
The 'almost' in the previous paragraph is there to allow for the fact that a few, very complex documents (e.g. which use multiple advanced macros) might not be fully compatible between the two programs. However, you could probably use Open Office for many years without ever coming across such a document.
You can download Open Office from here:
http://www.openoffice.org/
Chris