Physical Memory in Windows XP

In System Information it lists 256mb for my total physical memory and 79.77 as my available physical memory. Why the difference? Is this the same as system resources in Win98? I have a customer who is running WinXP and their machine is running very slow, we’ve checked for virus, cleaned up the disk (all that that entails to run maintenance on it)and they have listed 128mb memory but only 12mb available memory.