Virtual Memory fills while idle

I run Windows 98 and if I start the machine, open my mailer and browser and then go away for a while, say an hour, Windows eventually, a bit at a time, transfers ALL of the RAM contents into the virtual memory.
When I then sit down in front of the keyboard, the system is so slow to respond to anything – and it stays that way – that I have to reboot.
I find this irksome to say the least.

How do I fix this errant behaviour ?

Further info:
While my drive C: has over 350MB free, windows will often fill it with over 250MB of swap file ! which then gives me “Your disk C: is running out of space” error and an offer to delete files.
This is obviously directly related to the first problem – a problem with virtual memory.