Select all of the range from A1:F (last row)

In 2003:

Go to Format|Conditional Formatting

Select Formula Is from Drop down and enter formula: =$C1="Y"

click Format and choose from Pattern tab.

In 2007:

Go to Home|Conditional Formatting|New Rule

Select "Use a formula to determine which cells to format" from top section

Enter formula: =$C1="Y"

Click Format and choose from Fill tab.