You should use a Dell Reinstallation CD/DVD. I think you should be able to get your hands on one of these with at least one of the systems. If not then you will have to get one on eBay. Such as here or here. They should all have either a XP Home or XP Professional COAs. To be legal you must use a Dell Reinstallation Disc of the same version. i.e. you cannot use XP Professional on a system with a XP Home COA and vice versa. Actually both discs will install on the machine and unlock/activate within the Dell BIOs without the need to input the key which is useful if some are faded. Since you are purchasing 40 it is likely to be from a large company and probably will have XP Professional. Once you have a disc you are probably going to want to make several duplicates so purchase a bunch of blank CDs. First you will need to bring it up to Service Pack 3 and then create a bootable .iso. Otherwise you are going to have to install the Service Pack via Windows Update on every machine. Burn this image to multiple blank CDs so you have a reinstallation CD for every system say. This can all be done by using 2 free programs nlite and imgburn. I have wrote a wiki which takes you through this Make/Duplicate a Dell XP Service Pack 3, Windows Media Player 11 and Internet Explorer 8 Reinstallation CD. Once you have this disc follow my other wiki A Clean Install of Windows XP on Dell Systems (of course Step 1 and 2 don't apply to you). In steps 3 and 4 I have told you to slipstream these updates so once you have the disc so start on Step 5. Step 6 is optional but I recommend you carry it out on all your systems to ensure they are wiped and completely clean. Then follow right up to Step 12, check the list of programs I list, they may be useful for you. Step 13 is an optional step.
I am considering purchasing about 40 used Dell Dimension 2400. The HD's are erased. Each machine has a Windows CoA.   Can I just rebuild it with a Restore Disc? How do I deal with the product keys? Is there any additional expense?
Reinstall on used "Green" Dell Dimension