[strike]I'm sorry I didn't mean to scare you! Just thought it'd be good to know.
Windows is ridiculously expensive and I don't blame anyone for going with an OEM license, but if that's what you're going to do you should know about the implications.
Also, I believe you can actually get around this by selling the built system to for example, your mom, and then buying it back again for the same price...
Then again, I've never heard of a single case of Microsoft going after individuals using an OEM license for personal use. Why would they, it'd only cost them money and scare potential customers away. Microsoft prefers people buying an OEM license to people not buying a license at all.
Note: I'm not a lawyer or anything, so don't take my word for gospel.[/strike]
More wrongness. Sorry.