If you want to type a value such as 10e5, 1 p, or 1-2, and you do not want the value to be converted to a built-in number format, type the number as a text value. To type a number as a text value, use any of the appropriate methods below.
Method 1
Place a space at the beginning of the entry.
NOTE: This method does not work if the entry resembles a number formatted in scientific notation. For example, typing 1e9 results in a scientific number.
Method 2
Choose Format Cells
Click the Number tab.
Click Text, and then click OK.
This method allows you to type data in the selected cells as text. You must perform these steps before you type the numbers in the cells.
Method 3
Precede the entry with an apostrophe. For example, type the following:
’1 p

Avoiding Automatic Number FormattingIf you want to type a value such as 10e5, 1 p, or 1-2, and you do not want the value to be converted to a built-in number format, type the number as a text value. To type a number as a text value, use any of the appropriate methods below.Method 1Place a space at the beginning of the entry.

NOTE: This method does not work if the entry resembles a number formatted in scientific notation. For example, typing 1e9 results in a scientific number.Method 2Choose Format CellsClick the Number tab.Click Text, and then click OK.This method allows you to type data in the selected cells as text. You must perform these steps before you type the numbers in the cells.Method 3Precede the entry with an apostrophe. For example, type the following:

’1 p

Source: http://support.microsoft.com/kb/214233

If you want to type a value such as 10e5, 1 p, or 1-2, and you do not want the value to be converted to a built-in number format, type the number as a text value. To type a number as a text value, use any of the appropriate methods below.
Place a space at the beginning of the entry.NOTE: This method does not work if the entry resembles a number formatted in scientific notation. For example, typing 1e9 results in a scientific number.
Choose Format CellsClick the Number tab.Click Text, and then click OK.This method allows you to type data in the selected cells as text. You must perform these steps before you type the numbers in the cells.
Precede the entry with an apostrophe. For example, type the following:’1 p