Windows NT employs about 150 Stop Codes. However, you encounter the following Stop Codes most frequently. For a complete list of NT's Stop Codes, see the bugcodes.h file in the Windows NT Device Driver Kit.
This code is probably the most frequently appearing code, and it usually results from a buggy driver. The most common source of the problem is that the Virtual Memory Manager has detected a kernel-mode component's attempt to access pageable memory when the IRQL is Dispatch Level or higher and the memory is in the paging file. The IRQL must be below Dispatch Level for this operation to be legal. Look at the modules listed in the Stop Code and stack trace areas of the screen for a possible candidate. This code can also be a side effect resulting from a driver not shown in either area that scribbled on memory it shouldn't have.
UNEXPECTED_KERNEL_MODE_TRAP 0x7F and KMODE_EXCEPTION_NOT_HANDLED 0x1E
These two codes also show up frequently. In this case, the Microkernel's processor exception handler has detected that a driver or subsystem has tried to execute an illegal processor instruction, or a software instruction that NT cannot interpret. The cause can be a faulty memory module or a driver that has corrupted memory. The module information on the blue screen is usually misleading in this case, making it difficult to identify the source of the problem.
With this code, if you've added a new virus scanner or someone has accessed a shared volume over the network for the first time on the machine, the Server device driver can be at fault. The Server device driver constructs I/O request packets with a slot for every device driver on the path to the disk. Sometimes the number of I/O request packets the Server device driver allocates is insufficient, resulting in this Stop Code. Try increasing the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\ServicesLanmanServer\IrpStackSize setting to a number higher than 4 (or whatever it's set to) and see whether the problem goes away.
If you see this Stop Code, NT is very early in a boot and cannot access the disk partition that boot.ini is pointing to for the location of the system files (where your \winnt directory resides). The disk containing that partition is faulty, or the data on the disk or partition has become corrupt. I encountered this code when I left an NT 4.0 distribution CD-ROM in my CD-ROM drive and rebooted. The computer tried to boot from the CD-ROM, and NT displayed this message when it couldn't continue. An NT repair install is worth a try, but you'll likely have to buy a new driver or reformat, reinstall, and restore backed-up data.