The default login form in Joomla 1.5 is managed through a built-in module called Login Form. You can enable/disable this module from your Joomla admin area > Extensions > Module Manager.

In the Filter: field type Login and click on Go.  The Login Form plugin should appear below. To enable it, simply check the box next to it and then click the Enable button located at the top-right Joomla menu. You can disable the module in the same way, the only difference is that you should click the Disable button.

If you have problems with Joomla's login form, ask your host for assistance. SiteGround provides the best Joomla hosting and such requests are handled with ease.