Yes, fyrstormer is correct that the SSD drive needs software support (the "trim" command"), available in Windows 7 but not WinXP.
Note also that only certain SSD drives support the trim command, so you need compatibility both on the software and the hardware sides. There is an intel utility you can download free designed to measure specs for intel SSD drives, which can also be used with other drives to check them out for compatibility with the trim command.
The trim command disables defragmentation of the HD (which is not helpful for SSD drives) and tries to minimize delete/write cycles, since SSD HD's have an upper limit and their performance degrades over time as these cycles increase. Operating systems have an unfortunate tendency to keep writing/deleting info. on the HD, and the availability of the trim command indicates some effort to reduce these cycles.
Of course, if you are using an old computer and a new SSD revitalizes it, why not? You may not run up the cycles to the point where it becomes a problem and, in the limit, you could put in yet another SSD (kind of like replacing incan bulbs!) if necessary.
The disadvantage of the SSDs is capacity. I put in one as my main bootup device and then found I was running out of space, even though it was 128GB.
So you need a secondary standard HD with the large capacity if this is an issue with you. Put the minimum software necessary on the bootup SSD drive, and everything else on the large standard one.
One interesting middle solution is to use a flashdrive which can be used together with an existing standard (magnetic) HD to speed up the bootup process. USB flashdrives that are compatible are labeled readyboost or something like it. It has software support in Windows Vista.
Of course, one can also work to eliminate problems that are slowing up the bootup process, including cleaning up temporary files, defragmenting (standard) HDs, eliminating unnecessary programs and processes... regardless of the HD used.
Probably the biggest bottleneck for old computers is actually graphics performance, with the internet and youtube and what have you throwing graphics challenges every day, not to mention games.
Probably a good discrete graphics card, plus enough memory, are upgrades that can help revitalize a computer the most in today's environment.