#idClientLoginMainPage {
    position: absolute;
    left: 0;
    top: 0;  /* A unit of measurement is not required for zero length, zero is still zero regardless of the unit */
    width: 700px;
    height: 550px;
    padding: 10px;
    text-align: center;  /* You need this for IE 5.5, so it can stay, but that's not how you center a box
                            You'll need this for IE 6 too, if you're in quirks rendering mode.
                            You'll also find that all the text in all child elements is also centered
                            since this property is inherited */
}

#idClientLoginMainPage form {
    width: 600px;             /* Just made that up. */
    border: 1px solid black;  /* So you can see the dimensions */
    margin: auto;             /* centers the form */
    text-align: left;         /* Reset the inheritence of text-align: center; */
}

#idClientLoginMainPage {
    position: absolute;
    left: 50%;           /* offset from the left by 50%, step 1 in centering */
    top: 0;
    width: 700px;        /* Give it a fixed width, step 2 in centering */
    height: 550px;
    padding: 10px;
    margin-left: -350px; /* Half the width to center, step 3 in centering */
    text-align: center;
}