Hi, Cindy.
There's the geeky way to set the computer's internal clock in CMOS - and the
Windows way.
The geeky way is to press <Del> (or whatever key your own computer requires)
immediately after power-on or Reset and change the CMOS settings.
The Windows way is to right-click the clock (usually in the lower right-hand
corner of the screen) and adjust it there. This setting automatically
adjusts the setting in CMOS.
Does something else automatically change your clock's setting? Is this
computer on a network? It may be automatically synchronizing to the clock
in the server that controls the whole network. That doesn't sound likely,
though, if all the other clocks are correct.
My computer runs Automachron, one of several free-ware programs that
automatically goes online several times a day and synchronizes my clock with
a free "time server". WinXP (and Win2K, I think) has a built-in time
synchronizer, also reached by right-clicking the clock icon and choosing the
Internet Time tab. I suppose it is possible that some automatic
synchronizer on your computer is relying on an inaccurate clock somewhere.
If your time were off by exactly one or more hours, I'd suspect your time
zone setting, but that wouldn't seem to explain a 15-20 minute difference.
The headers on your post say that your setting is GMT -0700, which is
correct for Mountain Standard Time or Pacific Daylight Time in the USA.
(That's the proper setting for Redmond, Washington, which is Microsoft's
home, of course. Many users all over the world forget to change this when
they install Windows.)
I hope this helps. If you need further information, please tell us about
this computer's network status, and where (in the world) you are.
RC