The latest drivers from Synaptics, who are behind a massive proportion of PC touchpads in the market today, enable some multitouch functionality on older laptops that did not ship with such abilities.
The drivers were made available around 10 days ago on HP's website. Although intended for HP laptops, they work just fine on many other manufacturers' models — ZDNet UK has successfully tested the Windows 7 driver on a Dell Inspiron 6400 that came out just before Windows Vista was released.
Only limited multitouch functionality is enabled by the drivers — two- and three-finger scrolling is activated, but pinch-to-zoom is not.
The XP driver can be downloaded here and the Vista/Win7 driver can be downloaded here. Be warned, however, that the new scrolling functionality will replace the old: the squeezing-your-finger-against-the-side-of-the-touchpad method will no longer work.