If you only have 256 MB RAM I would use Lubuntu. This is a flavour of Ubuntu that uses the fewest resources. You may even want to try something else than Ubuntu.
256 MB RAM is really small. Even Windows XP (with all updates) would be really slow. Maybe you can try to upgrade it to 512 MB.
Also, some more information about your system would be nice.
To install it you'll have to use a CD or USB drive. I think a CD will be the best choice as older systems are usually not able to boot from USB.
You can download the ISO from the Lubuntu website and burn it on CD using ImgBurn (or another program that can burn ISO images). Don't just drop the ISO file itself on the CD as it will not be bootable that way.
Then you'll have to start your computer from the CD. This is usually done by pressing F2 or DEL just after you power on your computer.
Search for something called
Boot Order and make sure CD is on top. Then save and exit. Your PC will now start from CD.
Just follow the on-screen instructions. If you have any questions you can leave a comment.
Ubuntu has an internet browse (Firefox) and a office suite (LibreOffice: including Word and Excel) built in.
LibreOffice is not the same as Microsoft Office, but you can view and edit Microsoft Office documents with it.
I think Ubuntu also has an email client built in (Evolution) but I'm not sure about that. If it hasn't you can always easily install it using the following command in the terminal or use Ubuntu's Software Center:
sudo apt-get install evolution
I think you'll get used to Ubuntu faster than you think, especially if you're only using internet, email and office.
You can backup your important files to an USB drive by choosing
Try Ubuntu when you boot from your Ubuntu CD (do this before installing Ubuntu).
Ubuntu will start a live environment. Open a file manager and on the left you'll see all your hard disks. Click on
Windows to open your Windows files (the name may be different, just try them all).
Your files will be in
Documents and Settings/UserName. You can just drag them to your USB drive.