OK, this probably isn't the most efficient approach, but it's a relatively simple example:
<b>Excel 2010</b><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #BBB"><colgroup><col width="25px" style="background-color: #DAE7F5" /><col /><col /><col /></colgroup><thead><tr style=" background-color: #DAE7F5;text-align: center;color: #161120"><th></th><th>A</th><th>B</th><th>C</th></tr></thead><tbody><tr ><td style="color: #161120;text-align: center;">1</td><td style=";">Part Name</td><td style=";">Part No</td><td style=";">Data</td></tr><tr ><td style="color: #161120;text-align: center;">2</td><td style=";">Widget</td><td style="text-align: right;;"></td><td style=";">yada</td></tr><tr ><td style="color: #161120;text-align: center;">3</td><td style="text-align: right;;"></td><td style="text-align: right;;"></td><td style="text-align: right;;"></td></tr><tr ><td style="color: #161120;text-align: center;">4</td><td style=";">Part Name</td><td style=";">Part No</td><td style=";">Data1</td></tr><tr ><td style="color: #161120;text-align: center;">5</td><td style=";">Widget</td><td style="text-align: right;;">1234</td><td style=";">yada</td></tr></tbody></table><p style="width:3.6em;font-weight:bold;margin:0;padding:0.2em 0.6em 0.2em 0.5em;border: 1px solid #BBB;border-top:none;text-align: center;background-color: #DAE7F5;color: #161120">Sheet1</p><br /><br /><table width="85%" cellpadding="2.5px" rules="all" style=";border: 2px solid black;border-collapse:collapse;padding: 0.4em;background-color: #FFFFFF" ><tr><td style="padding:6px" ><b>Worksheet Formulas</b><table cellpadding="2.5px" width="100%" rules="all" style="border: 1px solid;text-align:center;background-color: #FFFFFF;border-collapse: collapse; border-color: #BBB"><thead><tr style=" background-color: #DAE7F5;color: #161120"><th width="10px">Cell</th><th style="text-align:left;padding-left:5px;">Formula</th></tr></thead><tbody><tr><th width="10px" style=" background-color: #DAE7F5;color: #161120">C2</th><td style="text-align:left">=IF(<font color="Blue">COUNTA(<font color="Red">A2:B2</font>)=0,"",IF(<font color="Red">COUNTA(<font color="Green">A2:B2</font>)=2,"Please enter one value or the other",IF(<font color="Green">B2="",VLOOKUP(<font color="Purple">A2,A5:D5,3,FALSE</font>),IF(<font color="Purple">A2="",VLOOKUP(<font color="Teal">B2,B5:C5,2,FALSE</font>),""</font>)</font>)</font>)</font>)</td></tr></tbody></table></td></tr></table><br />
If you want to post an example of your validation sheet and the data sheet someone can come up with some code for you. See the link in my sig for how to do it.