I too have contracted the highly contagious Guitar Hero Fever. After playing a whole bunch of it at Joe's place over Animazement weekend, I was hooked and had to get my fix. My only criticism of Guitar Hero was that there just weren't enough songs, and certainly not enough of the songs I would have liked to see in the game. That, as I understand now, is pretty much everyone's dilemma.
The solution to this problem is to grab Frets on Fire for PC (also available for Mac and Linux), which is a free open-source GH clone. The best part is that you can import all of your Guitar Hero I and II songs into it. Simply throw your game disc into a DVD drive, and let Frets on Fire churn away. Just be prepared for it to take a few hours.
I found FoF to be a great introduction to GH gameplay as well, allowing you to try out this type of experience for free. Initially, Frets on Fire is set up for the keyboard, which you must hold upside down and use much like a guitar. This is totally awesome in itself, and I will have none of your GH purists' complaints about it. However, if you want to maintain your GH skillz, there is an alternative.
If you don't have a guitar controller, you can certainly buy one to use with Frets on Fire. I opted to get the Nyko Frontman wireless guitar controller. After that, you just need a PS2 to USB converter, like the one offered by Radioshack. I can attest to this converter working very nicely with this setup, and it includes a driver CD for full support of PS2 peripherals. So yes, make sure you install those drivers.
Feel free to run the barebones FoF if you just want to try it out for a little bit. But if you want a better experience, I suggest delving into some of the mods that have been released for the game. The FoF community is basically in two camps. On one side, you have the people who still prefer the 1.1.x version of the game, running it combined with what's known as the RF-Mod. This combination is the most stable, and bug free, though it does lack some of the nice new features of the latest FoF release. Which brings us to the other side, being the players of the 1.2.x version (latest release), now significantly enhanced by Raph's GH-Mod. This version has a few stability issues, and may be a little slower on systems running dual-core processors (like mine). Even so, I prefer the latest version due to some nice added features and slightly different overall feel to the game. For big GH fans, this one may be easier to acclimate to.
After you have picked your version, and installed either the RF-Mod or GH-Mod (which is a must), you're all Ready to Rokk. Moreover, you can now get literally hundreds of additional songs to play around with. And if that's not enough for you, fire up the FoF editor to fret your own tunes. There may a bit of a learning curve, sure, but you're not going to get this opportunity with Guitar Hero.
Another great feature of Frets on Fire, in case you're still debating, is the World Charts. Every time you play a song, the score gets sent to the charts' server and recorded. You can then see how much you rock, or more likely suck, against everyone else playing FoF in the world. Look up your own name (or mine) to see a record of all your scores.
If you have problems getting anything to work, or just want to chat with other FoF players, you can always visit the forums. Just try, for the love of the guitar gods, not to sound like a complete noob. No forum likes that, and I'm sure you can figure some things out on your own if you just try.
Other than that, you should be set. So get out your guitar controller, go get that converter, and be prepared to play more songs than you can shake a... well, a guitar controller at. See you on the charts!