XP is still available to buy for new computers, (Only the Retail versions have been discontinued by Microsoft) so you should be able to specify XP on any computers built by 3rd parties. (This also includes Dell)
For what you want your computer for, Vista will be fine, (Go for the Home Premium version) and you will not need a high end Laptop, just a simple one will do. (Once you get past the initial changes in Vista compared to XP you should get on with it quite well)
If you want to use your computer for music then get the most powerful one you can, (VSTs and music software eat up resources) and avoid Vista in any form. (It is a total disaster with music, no matter what version you buy)
Above all, enjoy your new computer.
Hope this helps

Bill