From a corporate, business point of view, when protecting company secrets, customer personal information, or a large network itself, fighting back against fraud, hackers, spam and spyware gets exceedingly difficult and expensive. Any serious hacker would tell you that no system is completely impenetrable, and that if a system is designed by humans for security, it can be broken into by humans.
We are getting better at protecting against spyware, as computer programs designed to monitor for it on a minute-to-minute basis have been quite effective. Hackers are not programs, and are more difficult to defend against. The US government has a habit of hiring these hackers out of prison so that they are working for them instead of against them.
PC's are difficult to guard against viruses because the Operating Software (overwhelmingly Microsoft) has to be written to cover a large number of different computer platforms, and there are gaps in this programming that hackers can exploit.
This is quite an extensive list of threats to computers that could be generalized. One of the smaller threats, but certainly the biggest nuisance caused by hackers, viruses, spyware, frauds, and spam is that it slows down your computer. You've probably noticed when you buy a brand new computer that several programs automatically pop up and run on "start up," causing your computer to take longer to boot up and get to your home screen. This is the same thing with the threats you have listed above. Most will not only slow down your computer at start-up, but will make surfing the internet slower and more tedious. Spyware often results in tons of "pop-ups" that when left unblocked can take forever to weed through and close.
The more serious threat posed by the above list however, is the stealing of your personal information without your permission, often without your knowledge. Spam is fraudulent solicitation that often asks for your name, email and address. Then, your information is sold to other companies to send more advertisements to your email and snail-mail box. Even more serious fraud includes having personal passwords, sensitive information, even banking information stolen and actually being robbed of funds or your identity, electronically.
Everyone can take basic precautions to avoid these threats. It is well known that Mac computers are less susceptible to virus and ad-ware than PC's, as hackers are going to work the hardest to target the largest number of people possible, and PC users are still the majority. With any computer, you should buy virus protection (not simply take the program that comes free) and run regular "sweeps" on your computer for general clean up. Also be sure to install any and all updates immediately upon receiving them, because just like the flu virus, computer viruses are always changing.
Be very careful about how much of your personal information you have "floating out in cyberspace." Most people do not realize, for example, that every time someone clicks "like" on a Facebook application or ad, that company is directly connected to all the information available on the person's Facebook page. This is how Facebook seems to not only know all of your friends in the entire world, but all of your specific "likes" and hobbies as well. There has been speculation that Facebook is potentially crossing into illegal territory with its information sharing. Unfortunately, most users are "willingly" offering up the information without even knowing it.
Encript your passwords with numbers and/or symbols and change them regularly. Do not ever use a debit card to pay for something online. Credit cards have built in fraud protection and are not directly connected to your bank account. Finally, if something looks phishy, it probably is. Trust your instincts. Never open emails or download anything from a person or site you do not know and trust.