Linux live cd's for you:
ubuntu:
http://www.ubuntulinux.org, download desktop live cd edition
even better,
backtrack:
http://www.remote-exploit.org/backtrack.html
Download the cd image and burn to a cd.
Go into bios settings, change boot order to cd first, and boot into backtrack. choose the default option first. If that doesn't work, try the VESA mode. If that doesn't work, use something different. From the desktop on backtrack you will be able to (barring encrypted hard drive) access all files on the hard drive. Go into "computer"(? can't remember what it is in kde. should be something like computer). The icon in the window that pops up should say "hard disk drive" or "sda" or "hda". From there, go wild. if you go into ubuntu, go to places>computer and "(size of drive here) volume". Either way, you will have access to all (unencrypted) files on the computer. This is good for just history and things you can see through files, but look at the next part for other stuff
Also, I think backtrack can do sam cracking but im not sure...
P.S
Just saw your post on vista confusion. If the computer is left on again, click start, click on the picture in the top right of the start bar (should be sticking out of the start bar a little bit. That will take you to "user accounts". click in the bottom "Manage another account". if uac is enabled, it will come up. just hit continue. if its got a password ur fucked but i'll proceed under the assumption that there isnt. Click "create another account". From there it should be self-explanatory. make the new account an *****, make a username like RECOVERY or something inconspicuous like that. Click "create account". Click on RECOVERY on the users screen and click create a password. Create whatever password you want. Then just leave the computer alone. The next time you need to get on, just log in with that account. Have fun!