You don't say what version of Windows you are using, however if it is Windows 98 you can use the Windows Paint program or Internet Explorer.
For Windows Paint use File|Open and change the "Files of Type" to the format that you require. Windows Paint is normall installed in Programs|Accessories.
To change the default viewer to either Windows Paint or IE open Windows Explorer select Tools|Folder Options.
Select the File Types tab scroll down until you find the file type that you wish to change. Click Edit and in the Actions. Select the word Open and then Edit.
In the Application used box set the program that you would like to use to open theat file type.