it will actualy cause stability issues. win 7 needs a specific instruction set to be setup in bios for win 7 and vista... its called the high precision event timer (hpet for short). and it must match your o.s bit rate. ie be set to 64 for 64bit and 32 for 32bit.
if its not set wrong the system will randomly restart and lock up... so the best thing you can do is save the files you want on the old hdd, then format it and use it as storage only without any o.s on it.
That make it clear. The main reason I want to still use the old HD is because I need to use some of the software on it to finish some urgent work projects... do you think it would be somehow possible through the bios set up to still run the old 32 OS system at least for a while to conclude these proects before whiping it out and dedicting the machine entirely to the new system? Thanks again!