I have a couple of things to say on this. First of all, there is a program you can use to get the serial number from your existing Windows
installation. I've never used it myself, but it looks fairly simple to use. You can download it and find instructions at this website:
ProduKey. As far as I know, this program is free, and if some reason it doesn't
work, it seems that there must be other programs that do something similar. Also, you may want to consider using Boot Camp instead of Parallels. It
comes with the Mac OS and allows you to install windows on a separate partition. I'm actually writing this on a Windows XP partition now, and it
works fine for the most part. The only problems I've had are related to Apple's drivers for the trackpad, which are extremely sensitive and can
really make things a pain in the ass if you're not used to it. I doubt it would be any better with Parallels, though.
It is rather odd that Microsoft's online store only lists shipping as the only delivery option; when I set up Windows 7 on my friend's Mac in 2011,
we were able to order online and immediately receive the product key. There were multiple download options, and I don't know why that's gone. I
agree that the lack of flexibility is rather ridiculous. Many people really only need another license, and it seems like a huge waste and
inconvenience for both sides of the interaction to only offer a physical copy. In any case, though, try to get your serial from your existing
installation first, and if that doesn't work, you can always call Microsoft's sales line, explain your situation, and see if there are any other
options they can offer you.
Regarding the stability of Windows itself, I agree with the earlier poster. I often hear about people switching to Mac after becoming fed up with
various problems with Windows, but honestly, unless there is something wrong with your installation or configuration, and as long as you have
anti-virus/anti-malware software installed, you really shouldn't be having that many problems. I've had very few problems with Windows XP and
Windows 7, and usually when something does go wrong, it's because I've done something stupid or installed something I shouldn't have. Almost every
time I see a friend's Windows computer that is having significant problems, it's because they are either running the system with the installation it
shipped with, complete with tons of unnecessary bloatware and other crap, or they have simply allowed a bunch of stuff to accumulate through
indiscriminate downloading and allowing programs to install toolbars and other nonsense like that (and of course, viruses as well). A fresh, clean
installation (provided you can get your serial number) would likely do wonders to speed up your older computers and eliminate many problems that you
were having before.
(On a side note, Internet Explorer really is terrible software, and nobody should be using it. Anyone using it would be saving themself a lot of
trouble and making web developers very happy by switching to almost anything else, but that's a completely separate rant.)