Well, you can go to the M Audio website and check the minimum specs. In most cases, you can run Sonar, Cubase and Pro tools M powered with Windows XP, 1 GB of ram (Although 2 GB would be a great idea, ram is cheap these days). As for the Processor, I know some guys who are running all this stuff on a Pentium 4 with 1 GB or ram. As you said, you just want to record your guitar, although at some point you might want to add vocal tracks. Also check Digidesign to see if the Mbox Mini has an updated ASIO driver, in many cases this will solve your problem.

I used Sonar for years, and Cubase before that. I just find the new version of Pro Tools to be flat out amazing, and you might be able to get it running with just a little upgrading. Keep in mind you will need M-Audio hardware (Profire 610 or 2626, etc) to get it running, but its well worth it, as the sonics of both of these are WAY beyond the Mbox.

Hope that helps, PM me if you need help,
Ed