When you open up the Date and Time Properties in Windows XP and then click on the Time Zone tab, you might see only a single Time Zone (let's use Italy as an example). Now, assuming you're not in Italy, and if the map only shows Italy instead of the whole world, you can't correct it to display your true Time Zone (like Eastern Standard Time in the US). Why does this happen? Well, you're missing registry keys which contain Time Zone information.

The fix? If you have access to another Windows XP system, open its Registry Editor (regedit.exe), then export the following key to a REG file:

HKEY_LOCAL_MACHINE SOFTWARE Microsoft Windows NT CurrentVersion Time Zones

Then close Regedit.exe and copy the REG file to the computer you're having trouble with, then double-click it to merge the contents. If you have no access to another Windows XP machine, you can restore the missing Time Zones by downloading Timezones.reg (you can find it with a quick Google search). Unzip the timezone.zip files and extract the REG file to the Desktop, then right-click the REG file and choose Merge.