How do reset my compuer to factory without a cd

All versions of windows have whats called system restore.

Hopefully it was turned on.

Go to start button, all programs, accessories, system tools,
system restore, click on choose a different restore and then next,
now, on the next page click on the little box that says-show me all restore points, now pick the oldest restore point, and this will return the pc to
a state when it was new.

The Dell Dimension 5150 ships with the Dell PC Restore by Symantec.

IF, the harddrive has never been repartitioned, and IF, another restore program has not been installed, the Dell Restore partition is still on the harddrive.

To access the Dell PC Restore:

1) Turn your computer on.
When the blue bar with www.dell.com appears,
IMMEDIATELY press the CRTL key, and the F11 key, At The Same Time.

If you do not press the CTRL and F11 in time, let the computer load Windows.
Restart your computer, try again.

If Windows is not loading for you, then press the CTRL key, ALT key, and Delete key, AT THE SAME TIME. (Not shouting)

This will restart your computer.

After you have accessed Dell Restore on this screen, click on Restore.

On the next screen click on Confirm.

Should take about 6 to 10 minutes to restore your computer.

As said this will restore your computer back to the factory settings. Back to the day it came from the factory.

All of your personal documents will be Gone.
(My Pictures, My Music, My Videos, My Recordings, and any programs/applications you have installed, or downloaded, plus any personal documents you have created)