Only if you can tell me how long a piece of string is :eek:
To start of with, when you boot your PC, it goes through a process called poweron self test, POST. During this time, and before it thinks about loading Windows you need to press F8 feverishly until a list of recovery type options appear.
In this list is Safe Mode, with or without extra options. Assuming you have got this far and it wont boot into safe mode from here, one of two things have happened. Either your OS Build is the proverbial knackered, or Safe Mode is disabled.
I assume you can get into Windows Normally, so I shall try and deal with the latter. Turn off, and on, press F2 lots, you will enter BIOS. BIOSs are different, but you may find a setting in there.
Also in 95,98,ME, using watchdog
http://watchdogpc.com/help-options-w...ironment.shtml
I spent a while on google, and can't find anything about a disable in XP, or W2k, if you have one of the above, then maybe you have a bad sys file, what OS are you running, why do you need safe mode?