How long has the operating system been running?
Over time, Windows builds up "registry" bloat.
If you have had the OS running for 2 years or more, formating the hard drive and re-installing everything will give you a noticable boost in performance. BACK UP EVERYTHING FIRST!
Some peers recommend wiping the hard drive and starting over fresh every 6 months.
Other "software" ways to speed things up;
Reduce the number of background applications that are running.
Disable every applicationin the system tray that is not "critical" to your needs.
If you have W98, run msconfig, and disable as many other startup applications as practicle.
Clean out your recent documents, temp and temporary internet files.
Disable find fast, and any other file indexing services.
Disable desktop themes, animated cursors, and sound events.
For the internet, use a compact browser in place of IE such as K-Meleon from http://kmeloen.sourceforge.net
From the hardware side:
From the performance tab of device manager;
Disable floppy seek
Set your system configuration to network server
Make sure that acceleration is on full for everything.
Sometimes, updating the motherboard drivers can improve performance.
I know you don't want to upgrade. However, I recommend adding more RAM to your system.
Increasing RAM to 128-196MB (add 64 or 128MB) will make a major impact for you.