If a repair is what you're trying to do, there is no substitute for the Win XP SP3 disk.  You can perform a clean installation with the Win XP SP1a disk, but then you will need to also reinstall your applications and restore your data from back-up media.

Have you tried contacting Dell Tech Support, or if in the U.S., the on-line form to request a replacement installation disk?

Have all cds that came with system when bought.They will not work because my unit crashed last year and dell out of warranty dept. loaned me a cd to install in system.That cd was a xp with sp3,so now old cds wont work because os in unit is newer than cds i have.And if request copys they will probably send same cds i already have.Any thing i could try sir?My unit will not open internet explorer,help & support,search,& internet options.Tech said have four files missing so unit will not work till repaired with cd.

Agreed, in order to perform a repair install, you must have the Win XP SP3 CD.  The repair installation will correct problems with the Windows program files, but you must understand that a repair install makes minimal corrections to the Windows registry.  Your data and application files will be left untouched, but if the problems you are having originate in the registry, a repair install may not help.

In order to obtain the Windows installation CD you need to contact Tech Support or use the form found at the link I gave you in my first post.  Any Windows XP CD that Dell sends out should have Service Pack 3 included.  They will not send you a disk having only Service Pack 1a as they no longer stock them.

The Dell reps are normally very helpful, but if you have problems getting a Windows CD with Service Pack 3, you can still use the disk having Service Pack 1a to perform a clean installation.  I ran a Dimension 2400 for nearly eight years and restored it several times using the disks originally sent with the machine from Dell, plus a download of the Service Pack 3 update from Microsoft.  Trust me, your disks can still be used.

For additional information, see this page on the Dell site:  Restoring or Reinstalling Windows on a Dell.  There are four possible restore scenarios on that page.  Also, forum member natakuc4 has this page that provides detailed information on installing Windows.

Just to note there is a section on my wiki Windows Reinstallation Guide and Related Wikies/nLite: Update your Windows XP Reinstallation CD gives instructions in slipstreaming Service Pack 3, Internet Explorer 8, Windows Media Player 11 and SATA Drivers into your XP Reinstallation CD:

http://en.community.dell.com/support-forums/software-os/f/3524/t/19480753.aspx

This effectively gives you an XP with Service Pack 3 Reinstalaltion CD/DVD or USB.