Getting your Windows XP computer set up to work with Tibetan Unicode is not hard, though you may want to set aside a good half hour to read and complete all the steps of installing fonts, keyboard methods, Microsoft Office 2007, and a newer "uniscribe" text-layout engine (don't worry, we'll explain later about what uniscribe is). To make the process easier to repeat if you want to do the same on another machine, you may want to assemble the various software resources described here on an USB pen drive or blank disc. That way, if you want help a friend set up their computer, it will just take a few minutes.
Downloading Tibetan Unicode Fonts from websites on the Internet is easy. Use a search engine and enter "Tibetan Unicode Fonts Download" or something similar. You'll find sites such as THLib.org which list several fonts available for download.
You may also try to find a copy of "Microsoft Himalaya", which is the Tibetan Unicode font bundled in Microsoft Vista. The font is not freeware and is only properly licensed to customers of a Microsoft product containing the font (such as Vista). Legal questions aside, there is no technical reason why Microsoft Himalaya won't install just fine in XP.
Again, search on the Internet for Tibetan Unicode Keyboard Methods. THLib.org will have several links to sites that offer keyboards. Download several of them, so that you can try and compare. People tend to feel rather passionate about one Tibetan keyboard or another, so having several of them at your disposal means that you can help yourself or others to the input method most desired.
Most likely, you are accustomed to using Microsoft Word for typing your documents. Ms Office 2007 has built in support for Tibetan Unicode, and you will want to upgrade if you are using a lower version such as Office 2003. When working with Tibetan Unicode, Word 2003 had an irksome habit of automatically reformatting Tibetan Unicode content to an inadequate font (such as MS Arial Unicode). Not only has this problem been fixed in Word 2007, but other issues, such as proper line breaking at the tsheg mark has been addressed. Overall, Office 2007 has much better Tibetan language support, and you'll probably want to install it into your XP box.
Be aware the Office 2007 has a much different look and feel than previous versions. This is an attempt on the part of Microsoft to make the user experience less overwhelming. Some will find the new interface aggravating. But after a small amount of use, most users will concede that the real functionality of Office has changed very little.
Perhaps the more drastic change in Office 2007 is the shift to newer file formats. All of your older documents should open just fine in Office 2007, so don't worry there. But take note: Office 2007's new formats are not backward-compatible with other versions of Office, so be careful! For example, the new format of Word documents end with the extension .docx and not .doc. When handing a document saved in the new format (dubbed OpenOffice format) to a user of Office 2003, that other person won't be able to open it. (Note that there is a plug-in for Office 2003 that will allow you to open new Office 2007 formats, but most people won't have this plugin installed.)
You can save files from within Office 2007 in the older Office format so that users on a older version of Office can open your documents (You would specify this under the "Save As…" option). Saving in this legacy format, however, can lead to some minor formatting problems upon Tibetan Unicode text--after all, Office 2003 does not internally recognize the special needs Tibetan text. You can experiment to see if the older Office formats do an adequate job storing your Tibetan Unicode documents. You may find that the older formats are fine.
There is a great, free alternative to Microsoft Office called "Open Office." The latest versions of Open Office also have specific support for Tibetan Unicode script, but you'll want to be sure to upgrade to the newest "Uniscribe" in your XP machine (more on this later)