If you have more things started than you have RAM to support, Windows can sometimes crash. Since you didn't provide error messages or codes or how many tasks you were running, I can't be more specific. However RAM is cheap - I recently bought 8 GB for my laptop for $42. One thing you could try is the use of multiple tabs instead of multiple browsers.
This my issue. Im a auto action buyer and when i have 3 browser (IE, firefox, and Chrome)open at the same time it crash every minute. What it could be the reason? I have windows 7 4gb ram intel pentium 2.4ghz and a fast inernet conection.