Running Windows XP on the Mac just for VB.NET is going to be a waste of money. If you want to develop on .NET, just get a PC. It'll be a lot cheaper than buying Virtual PC and you'll get much better performance.
You might want to take a look at Mono or dot-GNU. AFAIK, these are the only two open sourced .NET runtimes around and I think Mono supports some sort of VB.NET.
Failing that, if you're just looking for a VB type development tool for the Mac, have a look at REALBasic.
You can get Virtual PC and do it on that. Albeit your compile time etc, will be slowed, but it should still work. When I was at the University, and had to build databases and frontends using Access and Visual Basic, I used Virtual PC as I had no windoze PC either. It worked fine. I still got my projects done. What you should research is how many semesters you will need to use VB.Net. I only needed Visual Basic for 1 class and after that it was only java, php, or another language of my choice, all of which can be done with a Mac.
No no no, the question is, how many semesters will you be programming in VB.NET. If you will be spending all 4 years at University working in VB.NET (I wouldn't think so but you never know) then you need to consider a PC.
If you only have 1 semester that you will use VB.NET then you can probably get away with using VPC to do your VB coding.
Do you already own a Mac or were you thinking about buying one? I would image that the .NET IDE would not be as nice to work in under Virtual PC as on a real PC.
Will you only be taking the one programming class? Or will you have more down the road? My college was all Unix programming, no Microsoft specific work, but if you're going to be using nothing but Visual Studio for development, go with a nice PC.
There is no way that Visual Studio will be in anyway useful to you if you are running through Virtual PC. VS.Net has so many things going on in the background (dynamic help, source completion, making your life easier, making your life miserable) that only a NEW PC (ie >2GHz) can handle that with any kind of responsiveness. It still slow at times with the brand new computers we got this month 3.2Ghz P4(ht)
I am in exactly the same situation at the university i attend.
I just do all my asignment on the uni computers!
Failing that buy a barebones PC and you should be able to get all the software you need from your university (WinXP Pro + VS.Net) with MSDN