The demo version was set to expire quite some time ago... Either format the system installing a new operating system or buy a registration key for Windows Vista. If you've got it, I would install Windows XP Professional (SP3 is preferred). If you do not currently have an operating installer disk, you can download some free ones such as these:Linux MintUbuntuFedora
As mentioned, they are completely free and do not require you to register them. To install them you will need to have a CD (or CD/DVD) burner. Afterward download the .iso image to the computer with the CD/DVD burner. Then use a program such as ImgBurn
to burn the image to the disk. Once done, stick the disk into the CD/DVD drive of the computer with the Vista installation. Then start the computer and use the boot option to boot from the CD/DVD disk (Generally a message during startup will read similar to "Press F12 for boot options." Press the key associated with the message continually (in this case, F12) to bring up the boot selection screen). Sometimes you will be asked to press any key to continue booting from the CD. As it said, press any key and you will boot from the CD.
Hopefully you will be able to resolve your problem. Just remember, installing a new operating system will erase ALL data on the hard-drive. So if you have precious files on the hard-drive, I would not recommend installing a new operating system.
Let us know how things turn out, or if you require any further assistance.