Your Windows Vista and XP system is already equiped to write in Latvian
the commonly used fonts (Ariel, Times New Roman, Courier etc) are Unicode fonts. Unicode fonts contain the letters and characters for all the different languages including Latvian. Microsoft have also provide all the neccesary keyboard drivers including Latvian.
Select 'Regional and Language Options' from the 'Control Panel'. In the Input language section add Latvian. Set the Keyboard layout properties to Latvian Latin or QWERTY. Set the default input language to Latvian. The Microsoft keyboard driver works by keeping the right ALT key pressed down and at the same time pressing the required letter. Press a for ā. This works fine for lower case letters but is awkward for upper case somettimes requiring three arms or a friend. A solution is to use the Latvian IT Unicode keyboard driver.