I wasn't sure where the cut off was, but...you really shouldn't be using an unsupported OS on the internet (and for that matter, be careful about transferring stuff on disks too!)
In the case of OS X, that means you really MUST have 10.6 or 10.7. In the case of Windows that means you really MUST have Windows XP or newer (through 2014 anyway). In the case of iOS it means you must have iOS 5...and of course in all cases, be sure to use Apple or Windows update to keep everything patched up.
That goes for third party programs too, like Firefox, Adobe's Flash, etc. (Flash is especially annoying to check to make sure you're running the newest, though you can check here: