Hmm If you are using linux on your old pc and wanna use ubuntu I'd recommend to use xfce ubuntu version (which is afaik called xubuntu). XFCE is a lightweight window manager, which means it needs much less ram than gnome or kde. The design of XFCE is close to Windows, so you wont have much problems with it.

If you like linux, I also would recommend to test out Linux Mint. Linux Mint is a distribution that is based on Ubuntu but offers a few more config tools, making it even easier to use. The standard Linux Mint is based on Gnome, but there is also a XFCE version of it.