Though nearly all reputable scanning vendors spell it antivirus, there's no real consensus on the proper name. Some users (and vendors) may spell it anti virus, or anti-virus, or AntiVirus with the V in virus capitalized.
The only problem with the spelling you choose to use has to do with search engines. And it's the same problem you'll encounter when entering any search phrase. The search engine will return best matches based on what is entered.
Try a search on "anti virus software" and you will get different results than if you type in "antivirus software". This brings up an interesting trick with search engines - if you're trying to tease out some information and keep getting the wrong results, try alternative spellings of the search keywords.
It is useful to know, however, that since most legitimate vendors do use one word - antivirus - instead of anti virus, the alternative spelling can be one subtle clue to help you spot a scareware vendor.