The firm said that 2,312 new articles of malware appeared last month, an increase of more than one-third since December.
The Sober worm, called W32/Sober-Z by Sophos, accounted for nearly 45 percent of all malware. However, its recent dominance as the most frequently seen type of malware is set to end, the firm warned, because it stopped spreading on Jan. 6.
The Nyxem or Kama Sutra worm, which first appeared on Jan. 18, accounted for 3.6 percent of all malware recorded by the company last month. The worm, which lures users into downloading it by offering what it says is pornographic material, was set to delete PC files on Friday, but has so far failed to live up to expectations.
Graham Cluley, senior technology consultant for Sophos, said viruses that use porn as a lure will always have some level of success.
"If you look at the rest of the chart, it's mostly made of old viruses," he said. "There will always be a portion of people who think with their trousers instead of their brains and they'll get their computer infected."
The emergence of the Nyxem worm also shows the importance of safe internet use at the workplace, said Carole Theriault, senior security consultant at Sophos.
"The rise of the Kama Sutra Worm also shows the importance of educating employees on safe computing practices – whether it's opening joke files, pornography or screensavers, there is always a risk of infection," she said.
Following Sober, the top five was rounded out by Netsky-P, Zafi-B, Nyxem and Mytob-BE in that order.
Mytob-FO, Netsky-D, Mytob-EX, Mytob-C and Mytob-AS rounded out the top ten January viruses, according to Sophos.