To be sure that you are practicing "Safe Computing", there are two areas that need to be considered:

System Security

Viruses, Worms and Trojan Horses
These are programs that have invaded your computer, without your knowledge, usually with malicious intent.  The problems they cause range from destroying data, clogging-up your computer, and causing your computer to stop abruptly.  Protection against these problems requires the installation of AntiVirus software and subsequently keeping current with anti-virus updates (preferably weekly).   This software regularly scans your computer in order to detect and eliminate any viruses that it finds.  There are various providers of such software, some of whom (such as AVG AntiVirus) is available free for non-commercial home use.  Two of the better known commercial providers are Symantec/Norton and McAfee.

Spyware and AdWare
These programs also invade your system without your knowledge and generally are more disruptive and intrusive than they are malicious.  Many of them monitor your use of the Internet and send information about your surfing patterns back to their masters who may then target you with unwanted advertising.  Others may insert unexpected toolbars into Internet Explorer, or change your default Internet home page setting to wherever their masters want you to visit.  Frequently your system will be slower when they are present.  Again, there are many providers of software that will monitor your computer and remove these pests.  Spybot and Ad-Aware SE Personal are two of the better known providers of free software for non-commercial home use.  As with anti-virus software, keeping current with updates is important.

Firewall Protection
A firewall is a piece of software that runs in the background and monitors communications between your computer and the Internet.  Its primary purpose is to ensure that only bona-fide programs are communicating, and it will block data that is being transmitted by an unknown or untrustworthy source.  If you are using Microsoft Windows XP, basic firewall software is included and you have a level of inbound protection provided that it is turned on.  If you wish to have more depth of protection, a product such as Symantec's Norton Internet Security (which includes  Norton AntiVirus) monitors both inbound and outbound communications.

Data Security

Disk Crashes
Your data is kept on the hard disk(s) in your computer.  These disks spin at thousands of revolutions per minute, a fraction of a hair's breadth underneath a rapidly moving arm that reads and writes the data.  If anything goes wrong at this high speed, you will likely suffer a "disk crash" which will render the disk unusable.  The disk is easily and affordably replaceable, but what about the data that is now lost?   
There are companies that will, for a fee, try to recover what data they can from the damaged disk, but there is no guarantee that it will be possible to recover what is most important to you.  

Viral Attack
Even if you do not suffer a disk crash, a severe attack from a virus or similarly malicious piece of software, may render your computer unusable to the extent that the only way to proceed is to wipe the disk clean and start again.  In this case the data on the disk will be lost.

Accidental Deletion
The third primary cause of lost data occurs when you might accidentally delete or overwrite a file.  In such cases, you may be able to restore it from the Windows Recycle Bin, if the file is there.  Otherwise, it may be irretreivably lost.

The only way to be sure that the data on your hard disk is safely protected is to regularly make backup copies to another hard disk (if you have two or more in your computer) and to backup to CD-ROMs (which should be kept in another place in case the computer is completely damaged from fire or some other cause).  If you do not yet have a CD writer in your computer, now is the time to consider adding one.  They typically cost about £25, or less.