Print Driver Isolation feature in Windows 7 and Windows Server 2008 R2 allows some of the print driver components to be executed in a process (or processes) separate from the print spooler. By doing this, any problems associated with faulty drivers are isolated from the print spooler service and’ll not cause it to fail. For those of you that’ve been around since the Windows NT days, you may remember how a bad print driver could bugcheck the system. Back then it was because print drivers were kernel-mode (Version 2) drivers. With the move to user-mode (Version 3) drivers, bad print drivers only affected the spooler, and didn’t bring down the whole system.  Aside from the overall print system stability improvements, PDI provides a means to isolate new drivers for testing / debugging without affecting the spooler and also makes it easier to identify which drivers have been causing crashes.

Full Article: Performance team