If you are on Windows ME, try using this program(FreeMeter) to try the steps Anubis suggested.
http://www.download.com/FreeMeter/3000-2086_4-103...1.html?tag=lst-0-1
Although this program just shows the total memory you have, but in your case, if even the Start button takes a long time to load, there should be a big difference during Plus! and MSN Messenger is loaded and not loaded. Try whether you can see the memory usage difference with this program first, and tell us the results...
And, perhaps you can try to use programs to free up your memory so your system can run much faster.
Softwares like RAMBooster can do it.
http://www.download.com/RAMBooster/3000-2086_4-14....html?tag=lst-0-11
EDIT: I am not working with these companies, so if your PC crashes and result in system data loss after using these programs, dont come along finding me!!
...But they shouldnt.