My SD card wipes out some pictures everytime I put it in the computer?

I have a Nikon D3000 and a 16gig SD card.
I took a bunch of pictures and then put the memory card into the computer to attach to emails and send to a friend. I didn't create copies, delete, cut or move a file. I safely ejected and inserted the memory card back in. Afterwards, Going through my pictures random ones are missing and there are quite a lot. Its happened to me before, but I never really made a big deal out of it.
I wanna know how and why these are lost and how I should recover them. Its happened on my computer and laptop, do you think its a virus?

Added (1). Haven't formatted it in a while. By doing so, what would happen?

Can't tell you how and why, not enough info.
Not likely a virus.
You say you didn't copy delete or cut… Are there any programs on your computer(s) that automatically import or index or Autoplay anything?
My guess would be a defective SD card if it does the same on two computers.
When is the last time you formatted the card in the camera?

It's just possible this may work on your computers, but it won't work on flash cards or USB's.

http://www.bleepingcomputer.com/download/unhide/

Hope this helps.

It could be a virus. You'd better do a virus scan on your computer.

You need to scan the sd card with some anti-virus software like norton.
http://www.norton.com

you may also try to recover your files by scan the sd card with picture recovery program
http://www.001-software.com/file-recovery/