Definitely go for more RAM - 512 is really borderline. I dread to think how much rubbish has built up after 4 years - CCleaner will probably spend several minutes just listing it all! Spybot's "Tea Timer" really slows things down, I've stopped using it as a result. Firefox will soon be using 200-300 k of memory, particularly if you have multiple tabs open. I close and re-open it fairly regularly, and the difference is immediately noticeable. You can run IE through Firefox with the "IE Tab" Add On, if some sites need it.
For what it's worth the laptop I'm currently using was taking a yawning 5 minutes to fully boot up from cold, after 2½ years use - I finally did a clean install with a genuine Windows disc and it now takes little over a minute....In fact it was never this quick even when new, so I'm not keen on using "built in" recovery partitions.