Well, depends really.
To start of with If you are serious, then get a good sound card with decent in and outs, i got an echo one, 2in 8out with a breakout box, just makes plugging everything else in easier. If you still want tolisten to cd's get a card that'll support the digital out from the CD drive, there are ways around this, but your machine may not like you much for it
Secondly equip yourself wit a pod, behrinhger v-amp or whatever, it'll do the job of getting a guitar sound, and unless you want to buy yourself a VHT or Mesa preamp designed for recording it'll be fine (but if you had that money, you'd be asking a salesman). If you want to do vocals, get a decent mike, I'm sure someone will be able to tell you about a good all round mike just in case at some point you want to mike up cabs as well as do voacals through it.
Once you've got this then get a stable operating system, basically don't even bother with anything less than WinXP or Win2k, preferably win2k as it is a bussiness operating system and tends to not crash anywhere near as much as the others. Plus it's been around for a while, so drivers are pretty good. XP will do, but I won't stray from NT based operating systems, maybe because of superstition
Next, and last step is to choose your recording program. Personally, I really like logic and find it ridiculously easy to use, it has all your built in sequencers etc just like most do. others to look for are Cubase, Sonar, Nuendo etc, but Logic is nice and simple, so I'd recomend that, 5.5 is the version to look for (the last one for PC)
And another thing:
Do a CLEAN Install on the PC you'll be using, it'll make life easier in the long run
With a simple setup like this you can get surprisingly good results, but don't use a shit soundcard, it'll make your life hell as ASIO drivers and the like don't agree with them and you'll find life frustrating...
Good luck!
James