It could be many things, but I would ask you to recall when everything (PDF's, CD's, flash drives, etc.) were working fine. Then I'd try to find out everything you installed (or uninstalled since then) with an eye to one of those being the offending party. Maybe one of those processes did not complete correctly. Every time you make a change to the system, it creates a restore point that you can use to go back to when everything was running fine. I'd probably start there.
Most viruses get on systems by the action of the user. I don't open email attachments unless I am certain they come from a trusted source. I avoid clicking email links. I suspect mail that contain such attachments or links may come from a compromised system, even if they say they are from a friend or family member. I don't visit seedy sites. I install Windows updates (2nd Tuesday of each month) the moment they are available, I run the OS firewall and the run in my router. I eat a lot of pizza.
When I say which OS is best, it is based on how I use my computers. What's best for me may not be the best for you. But IMO both Windows and MacOS are worthy options and a lot of the claims of one being easier or more secure or more stable than the other are based on old information. An interesting observation - the program on my Windows 7 system that crashes the most and causes me the most headaches is iTunes, an Apple program. Go figure.
My computer seemed to be working fine until I couldnít look at PDFís anymore. That might have been about a month ago, or a little longer, if I recall right. I only have a basic AVG program installed that I run about every week. Last week stuff started coming up on my computer that said viruses and Trojan horses. I removed them to the virus vault. Then the beginning of this week more stuff started showing up on my computer and some of it could not be removed by AVG. I guess I should have a paid AVG, Norton, or something else installed on my computer, but I donĎt, because I am not sure what is best. I did go back to May of this year and restored my computer to that date. I also did a restore again, but guess something was already in my computer.
I donít open any attachments from someone I donít trust, donít even play any music or movies on my computer, donít look at email that is from someone I donít know and donít visit seedy sites, so I am not sure what happened to my computer. Basically the computer was working just fine for about 2 years.
I really canít recall anything that I might have done wrong, except open pictures of my friends garden. He just got a new digital camera and never had one before, so I donít think anything came from him.
I also install updates to Windows each time they are available. I donít know how to run the OS firewall. I do have a firewall though. I donít have a router and my Ethernet cable is just hooked right into my modem, since I donít run more than one computer.
I appreciate you telling me what is best for you might not be best for me. That is an interesting observation that the program for i Tunes, gives you the most problems, being it is a Apple program.
I like the part about your post that you said you eat a lot of pizza.
Thanks for helping!