If you really want an internal one, they're not expensive to buy off t'internet, example
Amazon or
Amazon.
Have you used an internal one before?
I had nothing but trouble with mine (which was installed by the computer manufacturer) so bought a pair of homeplug (like these, although mine aren't Belkin.
Amazon) adaptors, which are much more reliable, and ideal for desktop computers.
USB Wi-fi adaptors (like the one WMR has) have normally been okay for me in the past though (it was just the internal one that I've ever had problems with).
If you're short of USB ports just buy a hub like this:
Amazon. Poundland often have them too. Just plug things like mice and keyboards into them to free up ports.