Personally, I have an AMD Athlon XP dedicated to WinXP and an Intel P4 dedicated to Red Hat. If you're going to dual boot, have win installed first, then install linux on a second partition and use GRUB (a boot loader which comes built into the installs of many distributions, including RedHat 7+ (before that it was only LiLo) - but grub is better than lilo).
In any case ... I've always used Red Hat and I'm totally happy with it. It's the best in terms of balancing ease of use with power. Slackware linux, on the other hand, is a real hardcore distribution - be prepared to put a lot of effort into learning it, but once you do, you'll KNOW linux backwards and forwards
now for Mandrake ... mandrake boasts the most easy to use linux system - never used it myself, but supposedly it's got all these wizards and lotsa guis to help you out - it's supposed to be for windows users who want to get their feet wet in linux
that's good and dandy and all, but the problem with mandrake is that it is based on redhat. basically all mandrake is, is a redhat core with extra guis and nicer graphics on top of it to make it easier to use (sorta where apple is going with OSX -> unix with a nice gui)
the problem with this is that mandrake is always behind redhat in releasing stuff ... for example, redhat will come out with a new version. then, mandrake will take that new version and work on it, spruce it up, and distribute it
basically, just the redhat CORE is used in mandrake
all of redhat's proprietary stuff (such as up2date for easy package updates) isn't used - mandrake has its own update services for that
now of course if you were to go with slackware, you'd have to do it all manually ... but slackware is for geeks anyway :o