Yes it is, one quarter of the Worlds PCs are still using XP because it is good.
That version of Windows is not being updated anymore though by Microsoft.
You can change Windows things out for aftermarket things, for instance the firewall was never very good, and the only IE browsers that will work on it are pretty ancient now so use one that is still being updated such as Firefox, and get a new antivirusl from Filehippo which usually have their own firewall built in which will automatically replace the Windows one;
http://www.filehippo.com/
Just make sure that nothing else is bundled with it.
To make sure that any non Windows programs are up to date you need to download Secunia, which is free and excellent, and is described in the article below, which is someone else`s opinion;
http://www.pcworld.com/article/2102606/how-to-keep-your-pc-secure-when-microsoft-ends-windows-xp-support.html
Your PC is likely to be rather old and might suffer some disaster such as a hard drive failure or a malware infection at sometime, so you need to have a system in place to restore it from that eventuality.
You can use an external drive to do that by using free software;
http://www.macrium.com/reflectfree.aspx
Download that and register it with your e-mail address and they will send you an e-mail telling you how to use it which you can print off for future reference.
You can create a rescue disc with it, or just save your Windows installation, or clone your entire hard drive.
Other people are trying to keep XP up to date, might be worth a shufti;
http://www.wincert.net/forum/topic/12020-windows-xp-service-pack-4-unofficial/
XP is being supported for at least another year at vast cost to the UK National Health Service as that is what they use, there is a modification that you can do to your PC to get those security downloads, but I can`t find it at the moment, might be worth a Google;
http://www.theguardian.com/technology/2014/apr/07/uk-government-microsoft-windows-xp-public-sector
Dunbar Pappy knows of it so you might search his answers.
Best of luck, regards, Bob.