Certain keys will not work when pressed together due to the way multi-key rollover is implemented.
There's no software solution as it's a hardware limitation - some keyboards support more keys simultaneously than others, but you can't change how your keyboard handles it.
Keys like Shift, Alt and Ctrl tend to be special cases - they are almost always used with other keys so keyboard manufacturers tend to ensure that they don't exhibit rollover problems.
As ayckoster points out, "gaming" keyboards exist, with higher-than-normal rollover counts. True "n-key-rollover" requires that the keyboard recognise all keys when pressed simultaneously, but a limitation in the USB interface prevents this over that interface requiring the use of a PS/2 connection in most cases.
Apparently, Cooler Master now have a keyboard that supports full N-key-rollover over USB, so possibly some sort of keyboard driver can work around the issue with USB's signalling.