|I would think either a bogged down PC or an infection. First go to start, accessories, system tools, disk clean up and clean out everything listed. Then go back in the same way but choose defrag instead of disk clean up. Then I would go to start, control panel, add-remove programs and remove programs that you recognize but you definitely do not use. Then go to task manager (control-alt-delete) (or download a copy of Process explorer from microsoft since it gives more info on running programs) and stop processes from running that you do not actually need all of the time. [You may later want to remove some programs from starting with windows]|
If this doesn't help enough, update your antivirus/antiSpyware program and run it's deepest scan or replace if outdated.
If that doesn't help enough, try running Memtest to check out your RAM. Also note that newer programs, especially newer web browsers now require more memory (if you recently updated) and if you do not have around 1GB RAM, consider adding more.