How long are you prepared to wait
I started in IT just over 10 years ago, failed my A levels, too much dossing.
Eventually got a job as an Office Junior in a printers. Small companies like that often have very few IT staff, so I managed to integrate my normal day of job adding stuff to excel spreadsheets and helping the 2 IT people out, usually just trying to fix someones connection to a printer or the network, then after 6 months managed to get a job in IT as a nightshift operator, doing backups, checking schedules ect. 3rd job was a Desktop Support role, very interesting but i soon got bored of it. Now I am with one of the biggest IT solutions suppliers in the country, yes a lot of people hate the company I work for, but I enjoy my job and am a Lead Technical Analyst now, in charge of various virtualised infrastructures.
Point is, I have no qualifications in IT at all, not a single letter to put to my name, my experience first off was just home build PC, pretty much like you. I have worked my way up and dont regret it at all.
My advice is to get yourself vmware player, download windows 2008 evaluation and install that, play around with it, install IIS and get some .NET web applications setup, you can get loads from hotscripts.com. Also setup and install AD is pretty easy to be honest.
When comfortable with that, try installing a few linux distros and setup a few web services on that. Also to note, Citrix is a big player too, and you can sign up on the site and download a 365 day trial of their full Xenapp software, took me about a day to install with no knowledge of it beforehand.
You will likely pick up networking/firewall skills as you go on. If your employer offers to get you a qualificiation then jump at the chance, never a bad thing for free training. Make sure you read the small print though, they sometimes put in if you leave within 3 years, you have to pay for the training.
Virtualisation is pretty big at the moment in my experience, so do atleast have a play at home with vmware and citrix virtualisation. If you are enthusiastic about it, then you really do pick things up fast.