Windows 7 Pro has an XP mode, which allows most software to run in an emulated XP environment. I have not used it but once or twice though. Without knowing what software you have that is incompatible, it is hard to tell if it would work or not. If you have already attempted to get it to work, then I guess you don't have much of a choice.
If your new pc's are from dell, you can downgrade to xp "legally" if the os you purchased was pro or ultimate. The XP OEM disks that came with your old Dell system will most likely work in your new Dell system (if that is what you have) but may be missing drivers, complicating the install process a little. If your new pc's are not dells, then you cannot "legally" install the xp from the old dell systems onto them.
Previous Dell OEM XP cd's did not require you to put in the product key, it is only used for unique tracking between Dell and Microsoft. I guess you will find out when you put the CD in. OEM install disks are typically tied to the manufacturer's BIOS information, so if it can't find DELL in the bios somewhere, it will not install.
http://www.microsoft.com/oem/en/licensing/sblicensing/pages/downgrade_rights.aspx