Windows 2000/XP/Vista: Our recommended configuration would be Windows XP with 256 meg of RAM. You're certainly not going to have any problems with that setup or anything better. ECMLink works fine with Vista and Windows 2000 as well. And, of course, more memory is always better. But the basic XP w/ 256meg of RAM would work very well for you.
Windows 7/8/10: The ECMLink laptop application fully supports Windows 7, 8 & 10. If your laptop is size appropriately to run either, then it's *plenty* big enough to run ECMLink.
Tablets: Most tablets are Android or iOS-based tablets. These will not run ECMLink. Microsoft tablets that have Intel processors and specify Windows (not Windows RT) should be fine.
Unfortunately, we just don't have enough experience using ECMLink with these systems to really recommend a specific configuration. We do know, however, that you'll need at least Mac OS X 10.4 (Tiger) or Linux kernel v2.4.20. You can actually use an older Linux if you want, but you'll be required to manually build and install the device driver from source. Mac OS X 10.4 is required because that's the version Apple introduced the ability to specify a custom baud rate (required to talk to the ECU).