PC Reboots When on Internet connection

Hello,

My personal PC has developed a problem rebooting when logging into Outlook Express and downloading my e-mails and also when changing from one web page to another. Seems to happen twice before settling down when down loading e-mail, but also happens randomly when surfing web pages.

OS: Windows 98SE
No new hardware has been added.
Windows Critical Updates downloaded along with hangup on shutdown update (does this once and awhile anyway).

Thank you.