Are you referring to a piece of software called "Guitar", or do you mean an instrument?
If the latter, you need a good duplexing soundcard (SB Live is reasonable, but Event cards give much less latency).
In fact, depending on what you want to do, any soundcard with an audio in should give basic results. Just get a converter from 1/4" Jack (or whatever your DI out on your amp is) to the input jack on your soundcard.
Let us know more about what you're trying to do, and maybe we can solve this.