As far as I am aware there is no way to do this as the ram doesnt appear as a drive to the system. There may be a way. There was an application to load windows to ram, which speeded boot time and the like drastically. However I dont think you can use the spare ram as you want. It would be a good idea and probably can be programmed to make it happen but I know of nothing.
This may be of interest to you.
For XP and 2000 only
Go to Start -> Run and type regedit. From there follow this path. HKEY_LOCAL_MACHINE -> SYSTEM -> CurrentControlSet -> Control -> Session Manager -> Memory Management. Right click on the DisablePagingExecutive DWORD value, change its value to 1, save and reboot.
This will load windows to the ram completely. However I dont know of one for 98.