Yes, you definitely need an anti-virus program running on your computer.
The anti-virus program should be one that continuously is monitoring your machine and checking new files. If you buy an anti-virus program it will do that. If you get the free version, it probably will but Malwarebytes and possibly other free versions just works when you tell it to do a scan.
The anti-virus program will find and remove viruses and related destructive software from your machine you may see called viruses, worms, Trojan horses, etc. Some of the program now say they offer ransomware protection also.
In addition to viruses there are spyware programs that can get on your machine. Most anti-virus programs claim they get rid of this also, but my experience is they are good at virus-type malware but not spyware. Spyware is usually not destructive to your machine but spies on you. It may watch to get your credit card number, put lots of pop-up ads on your machine, hog your memory and slow the machine down.
The following are my suggestions for free anti-malware programs based on my experience.
I use Avast currently for anti-virus, the free version. I have liked it. There are other good ones out there. I have used Norton and AVG in the past. Norton has no free version, but I watched it save me several times. As with almost all software, they did release a couple bad versions. McAfee, I do not care for; I have seen it let things through that others caught, including I Love You back about 2000. Malwarebytes is good but to always monitor the machine you must buy it. Windows Defender comes free with Windows, but I have never seen it catch anything and it is content with other anti-virus programs. McAfee is not and some are not.
As far as spyware the only program that is just for stopping it that I know about is superantispyware (not related to superantivirus, which is a bad program) and it does good. One warning: the first time you run it the program may find 1,000 sets of spyware. Do not panic, that is normal.
In all these cases if you get the free you will have to agree to updates when available and if you buy, they normally update in the background.
Out-of-date anti-virus programs are worthless.
Keep your machines clean and have safe computing.