OK, this is another 'posting on someone else's behalf' post, so I hope this doesn't prompt any tricky questions!
So, the wife wants to install Visual Studio .NET 2003 on her machine (rather, wants me to install it!) but she already has VS2005. The reason is that she wants to replicate her setup at work (they have both installed for various reasons) and also because she has a book to work through that has all the examples worked through in VS2003.
So it seems that it would be possible to have the two installed, but is there anything that we need to look out for at installation time to insure that there are no problems (e.g. older files overwriting newer files). I think it's more the case that she wants to be able to develop .Net 1.1 and .Net 2 on the same machine.
They can definitely co-exist side by side, but install order is a better question. I have scars from getting 3 or more versions of office to co-exist, and the secret always was "install in release order." That said, VS.NET seems to be very well behaved, and I have not heard of any horrible problems installing 2003 after the fact.
Another decent option would be a 2003 Virtual Machine.