I'm sure
Ubuntu would work and it is very user friendly, but I think you'd find it a bit sluggish on older hardware.
Xubuntu (Ubuntu with XFE desktop) may be ok or it may still be a little too heavy.
I've heard lots of good things about DSL (as suggested above) but I haven't used it myself.
I've found
PuppyLinux a nice little distro with a warm helpful community. It runs nicely on minimal hardware. Puppy by default runs in live mode (off CD or USB, or combo) although you can install if you want.
I also heard of an alternate lightweight distro called
WattOS (its based on Ubuntu) although don't know what its like.
PS I would suggest set
Abiword as default for doc editing (lighter weight than OpenOffice - installed with many distro by default, otherwise should be in repository) and perhaps
Opera may be the best bet for web-browsing (I've heard its the friendliest to older hardware - if you use Firefox, make sure not to have too many extensions/add-ons loaded).