To change the way your computer displays the time
Open Regional and Language Options in Control Panel.


On the Regional Options tab, under Standards and formats, click Customize.
On the Time tab, specify any changes you want to make.
If you do not see the format you want in Time format, follow these guidelines:

To Do this
Display time in a 24-hour format Type uppercase H or HH for the hour
Display time in a 12-hour format Type lowercase h or hh for the hour
Display leading zeros in single-digit hours Type two characters, HH or hh
Suppress the display of leading zeros in single-digit hours, minutes, or seconds Type a single uppercase H, or lowercase letter, such as h, m, or s
Display a single letter to indicate AM or PM Type lowercase t
Display two letters to indicate AM or PM Type lowercase tt
Display text Type single quotation marks (') around text

Note

To open Regional and Language Options, click Start, click Control Panel, and then double-click Regional and Language Options



this is from help in windows xp btw.
hope it helped
peace