As the NCAA tournament tips off today, cubicle dwellers across the country will watch the college basketball extravaganza right on their PCs while they “work.”

Great for college basketball fans. Not so great for the folks who run their companies’ computer networks.

Companies are already struggling to keep up with the explosion of video being watched online at work. Such multimedia applications are major bandwidth hogs that can dramatically slow the performance of corporate networks. The start of the college basketball extravaganza will only compound that problem when employees watch live streaming of the games from around the country.

The increase in video-watching has already led a growing number of companies to restrict or block access to sites that offer things like video sharing, video streaming and other streaming technologies like Internet radio, according to industry observers.

“We think it has become a high-priority issue for our customers,” said David Ulevitch, founder and chief executive officer of OpenDNS, a San Francisco company that makes network monitoring and management tools.

What IT managers see is being reflected on computer screens across the country. According to a recent report by Robert Peck, a Bear Stearns analyst, 75 percent of folks online now play videos, up from 60 percent a year ago. Those viewers spend on average of seven minutes a day watching video online.

But based on recent history, companies can expect a spike in that traffic Thursday. Round 1 and 2 are played Thursday and Friday, and according to past statistics from Nielsen/NetRatings, there will be a surge in visits to NCAA-related sites.

One of the biggest sources of that traffic will be CBSSports.com, which will be streaming each game for free. The site offered a limited number of VIP passes that promise better access to the video streams, which otherwise could require viewers to wait at times.

Ever since the Internet entered the workplace, companies have been struggling to find a balance between giving employees total freedom when they go online and making sure they’re not abusing the access.

“I think there are a lot of companies that want to limit the amount of bandwidth that video applications can consume,” said Chris King, director of strategic marketing at Palo Alto Networks. “I want my users to be able to do what they want, within reason. The non-work stuff is fine, as long as it doesn’t impact my business applications.”

Initially, companies focused on blocking things like adult sites and gambling. More recently, some companies have moved to limit or block social networking sites. But now video has become the focus, especially as more and more content — such as network TV shows — can be seen online for free.

A spokesman at CBS SportsLine said the company has heard few complaints about its service.

“Frankly, most of what we hear is almost entirely from newspapers,” said Jason Kint, a senior vice president at CBS SportsLine.

Nevertheless, the company is doing what it can: It has posted instructions for network managers to block the site.