If your system seems to be slow all the time, then the problem may be insufficient RAM. Windows XP needs at least 256 MB to run well. If the computer is several years old, you may be trying to do more than what the hardware is capable of. Consider upgrading the hardware (e.g. new and faster CPU, more RAM, video card, etc.), or possibly replacing the computer.

On the other hand, does your system only seem to slow down while performing particular tasks? Look at the hard drive activity light. Is the light steadily flashing while performing these tasks? If the answer is yes to both these questions, then you probably have a disk I/O bottleneck. This kind of performance degradation occurs with applications or programs that heavily rely on the hard disk, such as games, image editors, database, etc.