checkbox javascript help needed

I need a system in JavaScript where if you choose any 2 starred items from the list, the price for the printer reduces to £54.99. Also, if someone chooses the Upgraded CPU option, they are allowed to upgrade to Win XP 64 Bit Edition. Please help! Thanks in advance!

[code=html]<table border=”0″>
<tr>
<td>CPU to Dual 64-bit 2.66GHz</td>
<td><input type=”checkbox” value=”yes” name=”cpudual”></td>
</tr>
<tr>
<td>RAM to 2GB*</td>
<td><input type=”checkbox” value=”yes” name=”ram2gb”></td>
</tr>
<tr>
<td>RAM to 1.5GB*</td>
<td><input type=”checkbox” value=”yes” name=”ram1gb”></td>
</tr>
<tr>
<td>HDD to 250GB*</td>
<td><input type=”checkbox” value=”yes” name=”hdd250gb”></td>
</tr>
<tr>
<td>256MB Graphics Card*</td>
<td><input type=”checkbox” value=”yes” name=”graphics”></td>
</tr>
<tr>
<td>Monitor to 20&quot; <small>(graphics card required)*</small></td>
<td><input type=”checkbox” value=”yes” name=”20monitor”></td>
</tr>
<tr>
<td>Micro ATX case (silver cube)*</td>
<td><input type=”checkbox” value=”yes” name=”microatx”>

<tr>
<td>Win XP 64-bit (only with extended processor) </td>
<td><input name=”winxp64″ type=”checkbox” id=”winxp64″ disabled=”disabled” value=”yes”>
<tr>
<td>Printer</td>
<td>&pound;64.99
</table>
<p>
<input type=”submit”>
</p>[/code]

Also in #JavaScript _↴