The answer is yes, your OS can get corrupted by the memory.
In normaly cases, the system BIOS will prevent you from running unsafe specifications. In the earlier day, many of the systems did not read from the memory SPD chip, so it was more of a concern. Today, most Intel boards, and systems sold by DELL, Compaq and HP read directly from the memory SPD chip, it is very hard to corrupt the OS by the hadware itself.
Now, we see many enthusiast boards on the market that allow you to do all kinds of things. So the risk of damaging the system is comming back to this area of the market. Still, the BIOS normally will not boot unsafe specifications. However, it is possible to completely disregard the systems BIOS and run the memory at whatever speed.
Normal negative syptoms result in bad sectors to the hard drive, corrupted and unusable files, and in the worst case: damaged or unusable registry files and keys.
If you believe your OS has been damaged, you can always try to run a recovery program, though sometimes it is futile. The windows installation CD, however, comes with such a program to repair the OS for certain cases.
Also, it's a good idea to make regular backup files. And registry backup files before you experiment on a PC that has valuable data.
<font color=green>"Believe nothing, no matter where you read it, or who said it, unless it agrees with your own reason and common sence." -The Buddha
<font color=blue>AIM BrentUnitedMem