Comparison shopping on the web could be easier than ever with the flurry of new software programs designed to help consumers save time and money. But download the wrong software, and you could end up with some nasty viruses, not to mention having your privacy invaded.

Start by seeing what's been written about the program. If a free download claims to have been well-reviewed, for example, check to make sure those positive reviews actually exist, says Maxim Weinstein, the executive director of StopBadware, a nonprofit organization supported by Google, PayPal and Mozilla, that tracks malicious and misleading software. Unfortunately, no anti-virus software can catch everything, so it s important to do your own research, Weinstein says.

Even if a program isn t malicious, it could still raise privacy concerns. In order to work, online shopping tools have to gather some information on your shopping habits the question is, what do they do with that data once they have it? If you re like most Internet users, you don t bother to read the fine print in privacy policies before downloading new software, says Ryan Calo, who runs the consumer privacy project at the Stanford Center for Internet and Society.

A majority of people see the words privacy policy and they believe it means that the company has a policy of privacy, Calo says. In reality, most privacy policies are written to give companies broad leeway in what they can do with any data they collect on you, he says. The Stanford Center has developed a web site, WhatApp.org, where experts and users can review online and mobile apps for privacy and security concerns.

You may decide you re comfortable giving up a little privacy in exchange for some extra cash. InvisibleHand, a free browser add-on newly available for Internet Explorer, claims to have identified more than $52 million in savings for U.S. shoppers since it launched in 2009. Here is a look at what the competition is offering.

InvisibleHand

This browser add-on, just launched for Internet Explorer and also available for Firefox and Chrome, aims to do your comparison shopping for you. While looking at a Sony Cyber-Shot 12.1 megapixel digital camera on BestBuy.com, for example, InvisibleHand alerted us that the camera was available for $10.99 less on Buy.com and also showed us five other sites selling the same product. Don t focus too exclusively on price and ignore other fees like shipping or return costs, says Thomas Harpointner, the CEO of AIS Media, an e-commerce consulting firm. It s in the consumer s best interest to look at the total price of ownership of that product and factor in all the fees, Harpointner says, and sometimes the best deal is really at a local retailer.

The app can have limitations. It s not as effective if your search is broad say, digital camera rather than something specific, like Sony Cyber-Shot. InvisibleHand also only supports certain products and retailers, so it won t work for everything. Founder Robin Landy says later this year InvisibleHand will add more fuzzy matching the capacity to compare prices on products, like flights or hotel reservations, that are comparable but not precisely the same.

PriceStalker

This comparison-shopping web site offers a free Internet Explorer add-on. Highlight a product s name, right-click on it and you can pull up a list of similar products available on other sites. The best results, however, come from the site itself, not the add-on tool. Right-clicking on that same Sony Cyber-Shot camera, for example, turned up more expensive options, but clicking through to the web site found several offers that would save about $36 off the BestBuy.com price. Shopping comparison sites like PriceStalker draw about 22% of the online shopping audience, but many retailers still aren t aware of third-party sites like this, Harpointner says.

Bing Shopping

Microsoft (MSFT) new search engine offers another free Internet Explorer add-on that connects with its shopping search function. It works much like the PriceStalker tool right-click on a product s name to see sample Bing Shopping search results. As with the PriceStalker tool, the best results were found by clicking through to Bing itself. The right-click option turned up a Sony Cyber-Shot for $46 more than the BestBuy.com price, but a Bing search beat Best Buy s price by about the same amount.

Bing s other lure: cash back. With a Bing Cashback account, you could get 10% back on a $107.99 Sony Cyber-Shot from CompUSA.com. The Cashback program works with well-known retailers like Barnes & Noble (BKS) or Sephora, but in many cases you can t take advantage of it if you re using a coupon or shopping a sale. Keep in mind that if you re just savvy enough to look around a little bit you can get these discounts anyway, Harpointner says. About 60% of online shoppers start shopping in a search engine, but Bing hasn t been able to gain much of that market share, so essentially Microsoft is getting very creative to bribe people to use their service, he says.