A jacked up PATH has nothing to do with why you wouldn't be able to log in. That's a completely unrelated problem and you most definately would not have to reinstall because your PATH environment variable wasn't set right.
If you read the corresponding link I gave you on either csh or bash (assuming you use one of those.. most likely bash if you are unsure) it would have taught you exactly what the PATH variable does. To sum it up with my definition before I move on..
When you run a program from the command line by just typing the command name, for example, ls, it searches the directories stored in PATH for the program. If your $PATH has /bin /sbin /usr/bin and /usr/sbin in it, it will search it in that order until it finds a program called ls.
Now, as Tim stated before me, "net" isn't a command that's specific to linux. It must come in some package that either deprecated their "net" command in an upgrade and replaced it with something else, or the upgrade completely removed the package because of a security flaw or a package that your distrubution feels it no longer needs in favor of another package.. or at all. It depends what kind of "upgrade" you're talking about.
The first sentance in my last post was "Research setting your PATH variable." It wasn't "Add /sbin and /usr/sbin to your path". I wasn't expecting that to solve your problem, they were examples of what is in your average PATH variable.
I tell ya.. I've been a Windows guy for most of my professional life and yes windows has problems, I don't deny it... but at least if it was a problem I caused, I know how to fix it.
When you're more experienced with windows, *of course* it's going to be easier to to fix your problems in windows. It's ignorant to assume you can sit down at any computer and fix any problem because you can do it in windows. If you put enough time and effort into linux, you'll get just as fluent.
Nothing you're doing right now is "failing". These problems are nessicary to learn. Don't give up, and keep posting questions and you'll figure it out.
I started off a DOS/Windows 3.11 guy, tried linux for the first time in 1995. It took me 3 months to get it to recognize the CD ROM through on and off playing with it. After that it took me another month or two to figure out how to dial up to the internet with it. Support and "easy" programs were far less available then than they are now. Keep at it and you'll see what it's all about.