Start the computer, if it's already running, reboot it,
After the POST (Power On Selt Test) and before windows's loading, type several times the F8 (or F5) key,
The Windows Advanced Options Menu appears
Select "safe mode" using the up and down key,
Press "Enter"
Accept any question
Another way, using the System Configuration Utility
Start Windows
Go to Start Menu => Run
Tape msconfig and press Enter
The System Configuration Utility appears
Go to BOOT.INI tab
Check the "/SAFEBOOT" option
Save and restart the computer
Your computer will restart in Safe mode.
Don't forget to restore this value in the future.
Why Safe Mode
In safe Mode :
Windows start in minimum requirement (file system, parameters...)
Almost all pilotes will not loaded
All resident application won't start, virus including
WARNING
Pay attention, you cannot use your computer in safe mode as you use it in normal mode, safe mode is just for troubleshooting your system. You cannot connect with the Internet, you cannot play with your favourite game, you cannot enjoy music.
Link
Another best way for troubleshooting, use Windows Optimum