You can install programs onto an external hard drive, and run them on your PC. However, if you hook the external drive up to another PC, and run the program, there's a high chance of the program failing to run, or failing to run correctly, because there isn't the same operating system with the same data in the registry required for it to work properly.
Unless the program is standalone and doesn't rely on the registry (for example, media player classic and portable apps like portable firefox) then it probably will not work.
|Desktop PC| |Macbook Air 13" (Mid 2013)|
|Windows 8.1 Pro (x64)| |AMD FX 4100 Black Edition 3.6 GHz @ 4.0 GHz OC| |Arctic Cooling Freezer 13 PRO| |Gigabyte Nvidia GeForce GTX 560 Super Overclock 1GB| |ASUS M5A78L-M/USB3 Motherboard| |Samsung 840 Pro 256GB SSD SATA III HDD| |Kingston HyperX 8GB PC3-14900 DDR3-1866| |OCZ ModXStream Pro 700W Modular PSU| |NZXT Vulcan Micro ATX Case|