Yes, My computer becomes slow after a few hours of use when I used Windows 98. I had to restart Windows 98 after a few hours in Windows 98 to make Windows not as slow.
I notice sometimes Windows Vista and 7 also seems to slow down after a few hours of constant use, but it is not as common as Windows 98 when I used Win98.
It's not just the windows. Probably has more to do with the hardware, especially the RAM. (although it's probably best to stop using windows 98 by now...)
Adding more RAM will definitely give you more time until the slowdown starts, but you will always have to turn off the computer every now and then to free up RAM.
