"The instruction at 0x77c6fc47 referenced memory at 0x00000014, The memory could not be written
Click OK to terminate the program"
The clue is "referenced memory
at 0x00000014".. it's referring to a specific part of your computers memory called high memory, and that's normally reserved for operating system use. The Windows DEP (data execution prevention) program is what halts other programs that try to use high memory locations, and it's purpose is to work in conjunction with a DEP capable CPU to stop rogue processes from causing havok.. things like virii, spyware, even some rootkits. I can't be sure what's
causing that all of a sudden, but if it happens even once to another program then it is not a geck issue. You can disable DEP protection for a specific drive or program. Searching for DEP in windows help and support will tell you how to do that if you want to. Even something as simple as running those programs in compatability mode may help.
Also, when you say "saving, loading certain cells, and L4D".. those things all send up a red flag as being things or events that put your system under heavy load, even if it's only for a moment or two. Many issues (at first) only show themselves when you're going ballzout... things like heat buildup or a memory stick that has an intermittant chip on it, so keep an eye on things.