A regular CAT5 is usually called a "twisted-pair" cable because one pair of wires is twisted, so-to-speak: the pin that the wire originates from on one end goes into a different pin on the other computer, and vice-versa for the second wire... a "crossover" cable does not have the same pair of wires twisted; rather, they go "straight through" if you will.
Most modern Macintosh computers can auto-change this setting on the fly, negating the need for a crossover cable -- you can either use one or a regular CAT5 cable to directly connect two Macs that support this auto-switching ethernet thing. Older Macs cannot do this, and therefore need the crossover cable to function correctly. Since you're using a PC and we don't know wether the PC can auto-switch like the Mac, using a crossover cable will work no matter what.