It can be effective to tweet, blog, and contact these companies and complain about their endorsement of SOPA, as reflected in their Business Software Alliance (BSA) membership. Associations like the BSA exist, in part, so that companies like Apple and Microsoft can have someone do their political wet-work while pretending to be above the fray. Call them out on it!
They even have a mass-settlement-extraction campaign, though it gets less press because it's not targeted at average home users; they're smart enough not to send threatening letters to grandmothers whose grandkids might've installed a pirated copy of Windows on their machine. If they suspect a company is using unlicensed software from a member, they'll demand the company undergo a "BSA audit", which will work to correct the problem and reach a settlement. Of course, they aren't a law-enforcement organization, but they try to pressure companies to agree to a BSA audit through a mixture of claims that the company agreed to such an audit via their licensing terms, and through threats of lawsuits if they don't agree. There's a whole "BSA audit defense/compliance" specialist-lawyer industry around that.
To be fair, many of the companies targeted are not exactly innocent, but many businessmen who do have valid licenses are also worried about the expense/disruption of the process. Disorganized shops who don't have their licenses nicely indexed can end up in trouble, too, if they can't manage to rummage through enough cabinets to locate copies of their licenses (one reason some have resorted to physically attaching the Windows license to each machine, so it moves with it). It's sort of like being audited by the IRS; sucks even if you weren't cheating on your taxes, except it's weird that it can happen from someone who isn't even the government!
There were also some allegations (I believe settled out of court) in the 1990s that the BSA was using its enforcement threats to coerce companies into buying site licenses of its members, because having a site license would exempt you from the possibility of an audit.
On one hand there is a lot of pirating of antivirus software, and yet to some extent P2P does increase the need to have it installed on one's system (which may increase demand/business).
I feel like antivirus software is less piratable than a lot of things though - you need to constantly update through a centralized server which checks your credentials (which are inevitably false if you've pirated the software).
I hope Android will become a more full blown OS in the next few years, so I can replace my Windows computers for good. I don't want to support in any way a company that abuses so much the patent system, and is also supporting Internet censorship. I think that day will come sooner rather than later.
If you want a free and open source operating system, you have all kinds of options, including some very polished distributions of GNU/Linux. I moved away from Windows a few years ago, and I've never looked back.
