I am not aware of any changes in ComboFix which would result in it's inability to scan on older systems with older hardware. It will not run on Windows 2000 but since you have run it before, I am assuming you are running Windows XP, Vista or Windows 7.
As a general policy, Bleeping Computer does not offer advice
on how to run ComboFix unless we asked someone to run it or
if there is a problem with the computer caused by running the tool. With that said, there are circumstances ComboFix will hang
or stall at various stages
due to malware interference
, failure to disable other real-time protection tools or the presence of CD Emulators (Daemon Tools, Alchohol 120%, Astroburn, AnyDVD) so that it does not complete successfully. Also, depending on how badly a system is infected, ComboFix may take longer to complete its routine than it normally does or fail to run properly. While that is not normal behavior, it is not unusual. In such cases, it is helpful to know at what stage CF stalled/crashed and to provide that information to the Helper who is assisting you so they can investigate.
While our policy is not to offer advice on running ComboFix unless we asked someone to run it, we are willing to assist with resolving problems caused after using it and we are certainly willing to help with malware disinfection. If that assistance requires running ComboFix, you will be advised what to do in order to get the tool to run properly or investigate any error messages.
If you need individual assistance with a malware infection, please follow the instructions in the Malware Removal and Log Section Preparation Guide
starting at Step 6.