Mételo todo en otro div.
<table>
<div style="position:relative; overflow:scroll; height:300px; width:300px;">
<tr>
<td><div style="position:relative; overflow:scroll; height:20px; width:249px;">Campo1</div></td><td>Campo2</td><td>Campo3</td><td>Campo4</td><td>Campo5</td>
</tr>
<tr><td>Valor1</td><td>Valor2</td><td>Valor3</td><td>Valor4</td><td>Valor5</td></tr>
<tr><td>Valor1</td><td>Valor2</td><td>Valor3</td><td>Valor4</td><td>Valor5</td></tr>
<tr><td>Valor1</td><td>Valor2</td><td>Valor3</td><td>Valor4</td><td>Valor5</td></tr>
<tr><td>Valor1</td><td>Valor2</td><td>Valor3</td><td>Valor4</td><td>Valor5</td></tr>
<tr><td>Valor1</td><td>Valor2</td><td>Valor3</td><td>Valor4</td><td>Valor5</td></tr>
<tr><td>Valor1</td><td>Valor2</td><td>Valor3</td><td>Valor4</td><td>Valor5</td></tr>
<tr><td>Valor1</td><td>Valor2</td><td>Valor3</td><td>Valor4</td><td>Valor5</td></tr>
<tr><td>Valor1</td><td>Valor2</td><td>Valor3</td><td>Valor4</td><td>Valor5</td></tr>
</div>
</table>