Version: XP, Vista, 7
If you're struggling to read the text on your screen simply right-click the desktop and choose "Personalization" or "Properties". In Windows 7 click Display, or in Vista select "Adjust font size (DPI)"; in XP switch to the Settings tab and click Advanced.
Select a larger size to suit you and click OK twice followed by Yes > Close, rebooting if prompted. Certain programs will throw up warnings – in Vista and Windows 7 you can right-click the program shortcut and choose Properties > Compatibility tab, then tick "Disable display scaling on high DPI settings".