My experience with Windows based computers is that to keep them working adequately (ie. half speed or better) I need to reinstall Windows at least once per year - wiping everyting on the HD out.That is a common experience among Windows PC users, but it can be avoided by following some basic good practices. Admittedly, doing so requires time and attention that is mostly unnecessary with Mac's.
I have had numerous Windows XP computers in use in my house for many years, as well as one Windows 7 computer. Every one of them works very fast, very stably, and as well as when it was new. And I have never had to do a re-install of the operating system on any of them, even when the hard drive containing the operating system has failed (I use a drive imaging program).
For a summary of those practices, see my post here.
In this case, though, I do second the suggestion of a Mac Mini, unless the computer would be used for additional applications that would make Windows and/or a more powerful computer the better choice.
Regards,
-- Al