For what it is worth, and as I understand it...
.NET 1.1 is an independent framework with no connection to 2.0/3.0/3.5.
You cannot uninstall 2.0 or 3.0 and keep 3.5 as all are one framework. 3.0 and 3.5 are not full frameworks. They are extensions to 2.0.
.NET 4.0 is an independent full framework with no connection to 2.0/3.0/3.5.
If you Download .NET Framework 3.5 Service Pack 1 (Full Package) it will install .NET 2.0 SP2 and 3.0 SP2 as prerequisite.
So it may be the order in which you installed earlier versions is causing the problem?
How about trying to install a full 3.5 SP1 and full 4.0?
You could also try a Repair. (Control Panel / Add Remove / Repair)
Plusnet Plusnet Pro 21CN Netgear DGND3300v2 router
Windows XP Pro SP3