Press the "Alt" key on your keyboard and while keeping it pressed, type the combinations of numbers below. The numbers must be typed on the keypad or the letter won't work. The keypad is on the right of your keyboard (NOT above the letters on your keyboard.)

"Alt" + "0225" á
"Alt" + "0233" é
"Alt" + "0237" í
"Alt" + "0243" ó
"Alt" + "0250" ú
"Alt" + "0252" ü
"Alt" + "0241" ñ
"Alt" + "0191" ¿
"Alt" + "0161" ¡
"Alt" + "0193" Á
"Alt" + "0201" É
"Alt" + "0205" Í
"Alt" + "0211" Ó
"Alt" + "0218" Ú
"Alt" + "0220" Ü

Fancy way - takes more to set up but then is faster than above:

How to set your PC up to type accents:

Press START, SETTINGS and select "Control Panel"
Double click on the Keyboard icon
Select the "Input Locals" tab
"English (US) " is probably already there. You want to add English (United States)-United States-International. Sometimes you do this by double-clicking the English (US) language, or by right-clicking.
Exit by clicking the "OK" button until all of the options boxes are gone.

The keyboard layout will remain the same as in an English keyboard, but you will be able to type the accents on the vowels by first clicking the apostrophe key followed by the vowel you want to accent. Also, you will be able to write the ñ by first pressing the Shift and ~ key and then pressing the n key.
To write the "dieresis" over the letter u for "pingüino", you first press Shift : (colon) and then press the u key.

If it isn’t working, look on the bottom of the computer screen for a blue box that says EN in white. Click it. The black arrow should be on the International one and not the regular English one.

source: http://staff.edmonds.wednet.edu/users/gilbertk/