What processor? How much RAM?
This may come as a surprise, but Windows 8 *can* run more smoothly on old hardware than XP sometimes. As long as you've got enough RAM the processor shouldn't be a problem. My old laptop only got replaced because I wanted to play more games - I used to use it for web browsing, email and some web development and it was fine on a 1.3GHz Pentium Dual Core. It had 4GB RAM and an SSD, which makes all the difference - newer versions of Windows can use spare RAM as a cache for commonly used files to improve performance, and obviously an SSD will speed up any computer. I strongly suspect that adding an SSD and Windows 8 to your existing computer will breathe new life into it