In windows XP most of viruses spread through Autorun (Autorun.inf) Function in windows. It is better to Disable Autoruns in Xp. I am not sure about windows Vista but In WindowsSeven and above this feature is already blocked so you dont need to do anything. Ok Here are the Two Mathods that show you "How to block autorun files".
Mathode 1 (Easy)
Use A Software to Block Autoruns. I recommend Panda USB Vaccine
Mathode 2 (A little bit Difficult)
1. Type regedit in RUN or Startmenu searchbox and press Enter. It'll open Registry Editor.
2. Go to following key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
If the above key doesn't exist, create it.
3. In right-side pane, create a new DWORD value NoDriveTypeAutoRun and set its value to FF ((As Shown in Screenshot in the attachment)
If the above key doesn't exist, create it.
5. If you want to restore the Autorun functionality, simply change the DWORD value to given blow:
Windows 2000, Windows Server 2003: 95
Windows XP, Windows Vista, Server 2008: 91
PS) If you want to disable Autorun functionality for all users in your system, use the "HKEY_LOCAL_MACHINE" instead of "HKEY_CURRENT_USER" mentioned in Step 2.
