The nice thing about Macs is the pre-loaded software is stuff you can actually USE, and an absolute minimum of brand tie-in, corporate-partnership crapware that seems to come on every PC nowadays. From your comment about HP PCs I'm assuming you'd be a new Mac user--forgive me if I assume incorrectly.
The Mac mini will ship with Panther, the current version of the Mac operating system. It's nice.
It also ships with iLife '05, which includes iTunes for your music, iPhoto for your pictures (compared to the one-off photo software for Windows that ship with most cameras, iPhoto is bliss), iMovie for your home videos, iDVD if you want to combine all those things and make your own DVD, and Garageband if you want to create your own music.
It will also come with the newly announced iWork. Keynote is basically a PowerPoint replacement, and Pages is sort of a combination layout/word processor thing that will open and save documents in MS Word format if you need it to.
The Mac also comes with Mail, a decent email client with a trainable spam filter; and with the Safari web-browser, which has built-in popup blocking. Both of these do not have the vulnerabilities that hackers usually use to send viruses to everyone in your address book, etc.
The Mac mini may also come with a trial version of Office (which, unless you need Excel, you never need to even open) and a couple games.
In all, you get everything a basic user needs, right out of the box--and very little that you don't. It's one of the reasons I switched.