Unfortunately your machine is not up to the task of running Vista. However you could install an easy to use Linux distro. which generally demand less in the way of system resources, you do not indicate how much RAM so I will work on the assumption of 512mb which was the norm for a 750mhz P3.

If this is the case than a whole host of Linux distros. will operate with specs. with ease. distros. worth considering would be
MEPIS Linux 7.0 http://www.mepis.org/
Mandriva 2008.1 Spring http://www.mandriva.com/
Xubuntu 8.04.1 http://www.xubuntu.org/

All of these distros. are easy to install and easy to use and you would experience an improvement in performance as well.

LUg.