XP Services just don't disappear by themselves for no reason, so with absolutely nothing to go on (like what does 'missing' mean exactly, XP version, Service Pack, antivirus/sntimalware software, how the BITS Service came up missing in the first place,
what you have attempted so far, etc., etc., etc.), it is usually a simple matter to reinstall the BITS Service in Windows XP (if that's the only problem):
Here's how:
Before making any changes to your registry you may want to manually create a new Restore Point and make a copy of your registry with this popular free tool:
http://www.snapfiles.com/get/erunt.html
Open Notepad (and only Notepad) to create a new text file:
Click Start, Run and in the box enter:
notepad
Click OK to open a new Notepad file.
Copy and paste the following lines of BOLD text into the new Notepad file:
Windows Registry Editor Version 5.00[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\BITS]"Type"=dword:00000020"Start"=dword:00000003"ErrorControl"=dword:00000001"ImagePath"=hex(2):25,00,53,00,79,00,73,00,74,00,65,00,6d,00,52,00,6f,00,6f,00,\ 74,00,25,00,5c,00,73,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,73,\ 00,76,00,63,00,68,00,6f,00,73,00,74,00,2e,00,65,00,78,00,65,00,20,00,2d,00,\ 6b,00,20,00,6e,00,65,00,74,00,73,00,76,00,63,00,73,00,00,00"DisplayName"="Background Intelligent Transfer Service""DependOnService"=hex(7):52,00,70,00,63,00,53,00,73,00,00,00,00,00"DependOnGroup"=hex(7):00,00,00,00"ObjectName"="LocalSystem""Description"="Transfers files in the background using idle network bandwidth. If the service is stopped, features such as Windows Update, and MSN Explorer will be unable to automatically download programs and other information. If this service is disabled, any services that explicitly depend on it may fail to transfer files if they do not have a fail safe mechanism to transfer files directly through IE in case BITS has been disabled.""FailureActions"=hex:00,00,00,00,00,00,00,00,00,00,00,00,03,00,00,00,68,e3,00,\ 00,01,00,00,00,60,ea,00,00,01,00,00,00,60,ea,00,00,01,00,00,00,60,ea,00,00[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\BITS\Parameters]"ServiceDll"=hex(2):25,00,77,00,69,00,6e,00,64,00,69,00,72,00,25,00,5c,00,73,\ 00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,71,00,6d,00,67,00,72,00,\ 2e,00,64,00,6c,00,6c,00,00,00[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\BITS\Security]"Security"=hex:01,00,14,80,90,00,00,00,9c,00,00,00,14,00,00,00,30,00,00,00,02,\ 00,1c,00,01,00,00,00,02,80,14,00,ff,01,0f,00,01,01,00,00,00,00,00,01,00,00,\ 00,00,02,00,60,00,04,00,00,00,00,00,14,00,fd,01,02,00,01,01,00,00,00,00,00,\ 05,12,00,00,00,00,00,18,00,ff,01,0f,00,01,02,00,00,00,00,00,05,20,00,00,00,\ 20,02,00,00,00,00,14,00,8d,01,02,00,01,01,00,00,00,00,00,05,0b,00,00,00,00,\ 00,18,00,fd,01,02,00,01,02,00,00,00,00,00,05,20,00,00,00,23,02,00,00,01,01,\ 00,00,00,00,00,05,12,00,00,00,01,01,00,00,00,00,00,05,12,00,00,00[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\BITS\Enum]"0"="Root\LEGACY_BITS\0000""Count"=dword:00000001"NextInstance"=dword:00000001
Save the new text file with a .reg extension to your desktop or someplace you can remember with a name you can remember, something like:
fixbits.reg
After saving the file, close notepad.
Locate the fixbits.reg file on your desktop and double click it.
Alternatively, you can right click the fixbits.reg file, choose Merge or Open With... and select the Registry Editor or Import.
Respond in the affirmative to the question... Are you sure you want to add the information to the registry?
You should then see a message that the information was successfully entered into the registry.
Reboot your system and test to be sure the changes "stick" and see if the BITS Service is now listed and if it will Start (if it isn't already running).
You can remove the fixbits.reg file when you are done.