A hard fault is when referenced DATA is no longer in memory, but rather has been swapped out to disk (PageFile)
It is not an error.
But if you are getting a large number of hard faults, it would explain the app in question being a bit slow, as its reading from the disk more than memory.
More memory usually helps reduce Hardfaults.
The laggieness, or slowdowns may not be related to the hardfault issue however. It may simply be that the system is being taxed a bit too much.
Lots of open Programs, many writes/reads to the HD etc.
My guess is your memory is just fine itself, but testing it certainly would not hurt anything.
If you want to test the memory, to be safe:
Windows has a built in memory tester.
Hit your start Orb, and in the search box, type "memory"
The first result will be "Windows Memory Diagnostic"
A better, more accurate method is with Memtest86.
Please see this tutorial for more information: RAM - Test with Memtest86+