Hello Jakob99,
Have a look for Bricopack (Bricopac ?). Bricopack will install, with an uninstall option, a Vista like desktop/theme. There are two versions. One gives a slightly Linux look with the penguin, the other gives a straight Vista look. Install the lightest option as the full customisation in both may not be able to be uninstalled. The lighter/cosmetic option in both do not use any system/RAM resources. Both are good to "play with" and don't worry if both put a set of new .dll's in the System 32 folder. Uninstalling reverses the process. These packs may work for you, they may not but are a bit of a change in the XP look and a bit of fun.
Cheers from Aunty Jack.