Dell says I could not load WinXP Pro on Dell T300. Why not?

You need to use Windows XP Professional SP3 media dvd if you want to use your XP Pro SP3 product key. As far as I know, the SP3 product key does not work with the previous version of media (non-SP, SP1, and SP2).

Windows operating systems require a valid product key in order for them to work. You would have to purchase WinXP in order to get a product key. Unfortunately, WinXP is in its last days of its product lifecycle and will not be supported. You'd be much better off trying to install Win7.

If the laptop is new it should prompt you to create a recovery disc.
If it is old and you dont have a disc, you could download a copy of wndows from a torrent site, but use your own serial key.
It would have to be exactly the same version, if it is an OEM copy, tied to a make (eg Dell)
Search for Dell windows xp pro.
you are looking for something like these..
http://isohunt.com/torrents/?ihq=+xp+pro+dell

http://btjunkie.org/search?q=+xp+pro+dell

You will need a torrent client like Utorrent or limewire to download torrents

If you are reinstalling Windows XP from a NON-DELL disc, and trying to use the Dell OEM product key, it will not work.
OEM versions of Windows have a modified precopy2.cab and a retail version of XP will not validate the OED product key from the chassis. This is because the OEM media for Dell has a precopy2.cab that pre-loads the product key when the system is BOOTED to the OEM media (It also pre-activates the OS)
Your choices at the point are:

1. Re-install the OS from a Dell OEM CD (They can be purchased on EBay for around $20, and only work on Dell Systems)
2. Purchase a valid Retail Key from Microsoft

Is it possible that you have installed winxp twice? You say that you installed XP Pro sp3 was that an upgrade from xp home? if so then you will need to uninstall one or both versions of XP then install only one.. Make sure you backup any important files first though

windows is not loading properly.
if you're not worried about anything on the computer:
insert the recovery CD, boot the system to the CD, and reinsatll windows.

Get a copy of Dell OEM WinXP Pro. It will install without a serial number.

Right click on the icon you use to launch the application, goto properties and select the compatibility tab. The options available here should help you run much older software.

winxp pro. US$ 178.58474
winxp pro. full product US$ 402.18875

Enjoy

its oem, you either need IBMs restore cd for your particular laptop (model) or an xp pro (any service pack will do) OEM disk.
You can not get the OEM disk at best buy, circuit city target etc those are retail versions and will not work. Newegg.com or frys electronics are your best bet, but by buying it from them you are getting another copy of xp pro, just use the cd with the keycode on your laptop and then you can install xp pro on another pc using the code that came with the disk. MAKE SURE YOU ARE GETTING AN OEM VERSION AND NOT A RETAIL VERSION.

Some technical stuff:
 For the xp pro disk you MUST have the COA product key available, this is usually on the bottom of your laptop, for IBMs disk you just have to have an IBM WITH an IBM motherboard as without the motherboard it will not work. For example, the motherboard goes out and you replace it with a non ibm board it will only boot for 3-5 times, the ideaa is that this gives you time to get your documents off. This is per Microsoft agreement with ibm and all other Original equipment manufactures this include hp, dell etc.

sorry if this is too technical for you please let me know.