Stop doing what you're doing that causes the problem.
I know that sounds over-simplistic. (sorry about that)
First off, you said that you leave your computer on while you go away.
You should NOT do that. Those things do catch on fire, you know.
I've seen computers that caught on fire.....It ain't purty!
There's also three other things it's doing while you're away,,,it's drawing electrical power and running up your electric bill,,,it's generating heat that it then has to try to get rid of,,, it's sucking in dirt, that will clog up your cooling mechanism.
Secondly, if you've not cleaned out your PC lately, it may just be loaded up with dirt, in the CPU cooling fan and heatsink, the PSU and in the air vents. If your processor chip gets too hot, it will just shut down. Clean it out.
Lastly, there IS indeed a condition in windows 98 (more than XP) called a memory leak.
The only way to get wasted memory back is to do a re-boot on your PC.
Even during booting up, some things will run for just a few seconds, that will never give you back all the ram that they used. Then as the day goes on, the problem just gets worser and worser!
After a couple of days of being left on, your little computer will have NO ram available at all.
Then everything has to run right off of the hard drive, slowing your system down to a crawl.
Computers that absolutely HAVE to be on 24x7 to receive faxes, support a network, etc., should have extra cooling fans installed and be cleaned on a regular maintenance schedule. They should also be running a memory management program like "MemTurbo" to keep the ram refreshed and overcome the old "memory leak". They should also be on a commercial quality Surge Suppressor and battery backed up Power Supply (UPS).