How can I make a link from a <td> table cell

Yes, that's possible, albeit not literally the <td>, but what's in it. The simple trick is, to make sure that the content extends to the borders of the cell (it won't include the borders itself though).

As already explained, this isn't semantically correct. An a element is an inline element and should not be used as block-level element. However, here's an example (but JavaScript plus a td:hover CSS style will be much neater) that works in most browsers:

<td>
  <a href="http://example.com">
    <div style="height:100%;width:100%">
      hello world
    </div>
  </a>
</td>

PS: it's actually neater to change a in a block-level element using CSS as explained in another solution in this thread. it won't work well in IE6 though, but that's no news ;)

Alternative (non-advised) solution

If your world is only Internet Explorer (rare, nowadays), you can violate the HTML standard and write this, it will work as expected, but will be highly frowned upon and be considered ill-advised (you haven't heard this from me). Any other browser than IE will not render the link, but will show the table correctly.

<table>
    <tr>
        <a href="http://example.com"><td  width="200">hello world</td></a>
    </tr>
</table>

Use this code. It expands an <a> to fill the cell horizontally. To fill vertically, use the height property as well.

td a {
  width: 100%;
  display: block;
}

td {
  /* Cell styles for demonstration purposes only */
  border: 1px solid black;
  width: 10em;
}

<table><tr>
  <td>
    <a href="http://example.com">
      Hello World
    </a>
  </td>
</tr></table>