Microsoft is still up to their old tricks, installing unwanted software that’s bundled with other things you might need (or possibly even want – who knows). If you use Firefox, you not only don’t want the .NET Framework Assistant, you don’t need it (and it’s not compatible with the latest Firefox anyhow).
Rather than re-invent the wheel here, I’m going to send you over to Wyatt O’Day’s blog to get a simple step-by-step tutorial on how to remove this particular bit of bloat from your PC. If you aren’t comfortable editing the registry, get a geeky friend to do it for you; you can buy her a pizza or something as payment.
I have used this method for Windows Vista (64-bit), Windows 7 (both 32 and 64 bit versions) and Windows XP; it works. Which is more than can be said for the Microsoft .NET Framework Assistant.