The easiest solution would be to buy more RAM, I'd recommend at least 2GB for Vista - if your laptop can accept 3, even better (as some of the RAM will be used for graphics whereas your PC will probably have a dedicated graphics card). Visit
http://www.crucial.com/ and do their scan to see what you can use (you don't need to buy from them). You may also find that turning off some of Vista's eye candy (like the Aero interface and reflective effects) speeds up the machine without recourse to a RAM upgrade.
Otherwise: using XP would probably result in a faster machine, but you would be best off checking the manufacturer's site to ensure that they provide XP drivers for all the various hardware components (many do). Check your documentation - a few laptops come with the option to downgrade to XP built in, and some mechanism to do so.
Have you no discs at all? You may find that while you do not have a "Vista" disc, you do have a "recovery disc" that contains Vista along with drivers and software for your machine. Some manufacturers provide a "recovery partition" on your hard disc that does the same thing. Again, check the documentation.
If you do not have a recovery disc or partition but do have a suitably sized external hard disc (or fancy creating your own recovery partition) you could use something like DriveImage XML (it's free) to "copy" your existing laptop data in case you need to reinstall it.