How can I get my computer to use the Spanish Alphabet and Spanish accent marks and so on?

Elaborate system mod's to change language will be a long haul method, which you'd need to do and un-do repeatedly.

Have a look at Google Translate, which (although not perfect) can do this "on the fly" and as you need or want,

http://translate.google.com/

The uppermost toolbar has many Google services (requires log in to access some).

The first set of drop menus has the "From" which you likely want as English; and the "To" drop can be Spanish. (each of those can be altered to what ever needs you have)

Type in the field your Sentence/Paragraph/Novel and hit the "Translate" button.

The equivalent will appear on the right, which you can Copy & Paste using the mouse.

It also has an audio function key to hear it, but I've not tried it.

Note there's an "Instant" option on the bottom to turn on or off, and it has a QR image in the "Phone" link that you can put on your Mobile smart device.

AS always, Google is watching what you do, and probably recording it somewhere, so use with those Privacy concerns in mind.

You must change the language in the control panel of windows. What version on Windows are you using? You should be able to go in Control Panel, Regional and Language Options, and then switch the language there!