You will need to obtain the CD key that was used on the old PC, its probably stuck on a sticker to a COA somewhere.
You then install Windows on the new computer and you can call Microsoft and explain exactly what your situation is when it is time to activate. You have the right to have ONE and ONLY ONE current installation of Windows that belongs to you. They will allow you to activate it then over the phone.
EDIT: This is assuming it was a retail distribution of Windows.
Microsoft is generally cooperative with this sort of thing.
You then install Windows on the new computer and you can call Microsoft and explain exactly what your situation is when it is time to activate. You have the right to have ONE and ONLY ONE current installation of Windows that belongs to you. They will allow you to activate it then over the phone.
EDIT: This is assuming it was a retail distribution of Windows.
Microsoft is generally cooperative with this sort of thing.