Probably you don't have to worry about getting virused from a site like Project Gutenburg or manybooks.com or feedbooks. However, you should probably note that there are sites where you can get ebooks (or movies or music or anything else that can be digified) where the material is not legal and the sources aren't trustworthy. But then, those sites won't be advocated here and I don't think that type of site will advertise in reputable places, which means the sites will be far more difficult to find, and will almost eliminate the possibility that you just happen into one. And frankly, sites offering books that have expired into the public domain that are doing it for the public good (like Gutenburg) can't afford to give out virused material. They work on grants and volunteer donations (not to mention volunteer time) and don't need the bad publicity that would cause. And certainly it wouldn't help further widespread use of ebooks.
And DRM is simply a way for a publisher to manage access to digital content, to keep it from being disseminated to the detriment of the publisher. It's a little like if you buy a hard cover book: you can sell your copy, or give it away, or donate it to the library, but you can't expect to profit from it after that. You couldn't sell the book, then expect the buyer to pay you a percentage if they go on to sell it. CDs and DVDs are the same way. You own it, but you can't profit from it, and your rights as an owner are somewhat limited in the ways you can use it and view it.