Serial Number in System Info is Not Long Enough

Question

I have deactivated CS5 from Computer A, noting the System Info before deactivating, which included a Serial Number.
When trying to re-install CS5 on Computer B, the Serial Number is not long enough.
This CS5 is a student edition.

Answer 1

did you get the serial number from Help>System Info, where it says "Serial Number"?  because, if so, that's not the serial number.  In fact, it's not even close.  my actual serial number started with 1 while the "serial number" in the System Info started with 9.
when you deactivated from the old computer, did you check the box to erase the serial number from the computer upon activation?  If not (i.e., the serial number is supposedly still on the old computer), i've heard that if you start a reinstall of CS5 on the old computer, it will fill in the serial number at that screen.  I don't know this for sure because the OS on my old computer literally crashed in an irreparable way immediately after I deactivated CS5.
If it isn't a student edition (you must not have the box - the box would clearly say so), then the only way to get the serial number is to know the email address that it may have originally been installed under.  Even that may not work,  when I spoke to Adobe support (after waiting 3 hours on my digital phone), I gave then every email address that I could imagine having used and did not find the record of my initial installation.
Again, good luck