Ephexx,

Dell's policy is that they will replace missing/broken CDs within 30 days of the invoice date of the computer, after which you will have to purchase a replacement OS CD.

If the new motherboard is a non-Dell motherboard that may be the issue.  Dell OEM Windows install CD's will not run on non-Dell motherboards.  You wil have to buy a new (non Dell OEM) copy of Windows. This is according to many posts I've seen on the various forum boards.

If you know your XP key code, then just borrowing someone elses CD and when it ask for the Key Code enter it. You can use the activate program to activate it,but it will probly refuses that, (it might since the key code is for an OEM and the CD you borrow is not). It gave me a number to call MS and they help me out for free. I know, because I did this with my daughters computer. Told MS tech support that the OEM CD was damaged in my move, so I used the boxed version I had for another computer and just used the key that was on the other one. Hope this info does you some good.

I have used an OEM DELL XP CD on a non DELL Computer & it installed OK.
It required Activation as was a non DELL Computer , so the 1st attempt failed as it used the DELL XP CD auto installed OEM DELL GENERIC  Product Key, when the 1st attempt failed it showed a Windows where you could type in a new Product Key & Click "Retry" button.
I typed in the COA Product Key of the non DELL Computer & it Activated OK.