Software interactions are a significant source of problems; but these are inadvertant. Software attacks (logic bombs, Trojans, worms and viruses) are deliberate and can also be significant.
Software threats can be general problems or an attack by one or more types of malicious programs.
This category accounts for more damage to programs and data than any other. We're talking about non-malicious software problems here, not viruses. Software conflicts, by themselves, are much more likely threats to your PC than virus attacks.
We run our PCs today in a complex environment. There are many resident programs (TSRs such as a Mouse driver) running simultaneously with various versions of DOS, BIOS, and device drivers. All these programs execute at the same time, share data, and are vulnerable to unforeseen interactions between each other. Naturally, this means that there may be some subtle bugs waiting to "byte" us. Any time a program goes haywire, there's the risk it may damage information on disk.
There's the further problem that not all programs do what we hope they will. If you have just undeleted a file, you don't really know if all the correct clusters were placed back in the right order. When CHKDSK "fixes" your disk for you, you have no way of knowing exactly what files it changed to do its job.
Software problems happen and can be very serious if you have not taken appropriate action in advance of the problem.
These are programs written deliberately to vandalize someone's computer or to use that computer in an unauthorized way. There are many forms of malicious software; sometimes the media refers to all malicious software as viruses. It's important to understand the distinction between the various types.
Logic Bombs Just like a real bomb, a logic bomb will lie dormant until triggered by some event.
Trojans These are named after the Trojan horse, which delivered soldiers into the city of Troy.
Worms A worm is a self-reproducing program that does not infect other programs as a virus will, but instead creates copies of itself, that create even more copies.
Viruses An entire topic has been dedicated to this threat.
Software does more data damage then any other problem category.
Non-malicious software problems can be a significant source of problems and one should always know their computer's exact configuration to be prepared.
Malicious software falls into several general categories: