Or use the body onload handler as sos suggested.. It makes for a more manageable project if you use onload to call one onload function that does all one-time-only functions, rather than putting bits of script inbetween html elements..

To srinivaskota, if you have some condition that must be met to enable the button; and the condition is usually not yet met when the page is first accessed ( or refreshed ), i.e. the button should only become enabled as the result of some javascript code; set the button as disabled in the HTML, i.e.
Then, when the condition is met, set the element's disabled property to false. You'll still probably need an onload handler to check if the condition is met, if it's some kind of persistant condition.