Alright. If you have a large enough USB device (I need 150GB for my Steam stuff). Just copy your entire Steam folder over to it. After you have done your OS reinstall just copy the USB stick back over to your Hard Drive. Now it's only a matter of finding your Steam application and starting it. After you are logged in all your games are back, but the GFX/Game-Play/sound settings will revert to defaults. I think the backup gizmo built into Steam blows chunks.
Saved games you want to also backup. Bethesda savegames can be found in "...Documents and settings\My Games" for XP. Vista and Windows 7 should be in the sameish place as well.
don't copy all of it, that is really a waste of space, just copy your save games, unless you are slaving a hard drive, to do this you will need another hard drive, you don't necessarily have to buy one, you can just borrow one from someone, when I reformmated my pc I just used my sisters. If you need to know how to slave then you can look it up on google, its not necisarrily hard, just put the hard drive in and connect it (can remove pin in the imput, but you don't need to) and enter into bios and enable the drive. I have only done this twice, so I don't know that much about it.