really, for ANY useage. A part of the problem is that by sticking with an out of date browser, you will become stuck with out of date plugins such as flash, java, toolbars........which collectively are more targetted than windows.
So you have multiple vulnerabilities - which are not getting patched
Whats at risk? The machine could be rootkitted and / or zombied, used as part of botnet run by criminals for spam spewing / DDOS attacks / hiding kiddy porn / banking fraud (of you and others)....... I could go on.
The simple fact is that if you are vulnerable you WILL get hacked. You may notice if the machine grinds to a halt, but thereagain you may not