I think anyone who's ever used a computer for any reason has experienced what you experienced.
When the EQ folks tell you to upgrade to EQ7, they are not too subtly telling you that they no longer support EQ5.
The memory problem mentioned by one of the other responders to your post could be the culprit. I doubt that XP is the problem, unless you haven't kept it up to date with fixes, etc for a long time before Microsoft ended support for it. (We have an xp that hasn't been updated since 2003. You can reach the internet with it, but when you try to open a web page, there's nothing but a mess in front of you because the software is so out of date. We can't even get the microsoft updates because it is so outdated. It still runs cds/dvds etc that were created when XP was king, so we let kids play games on dvds from before that time on it, and it's still good for using Word and Excel.
If the problem is inadequate memory on the xp or the xp hasn't been updated for a long time, an update from EQ5 to EQ7 may help, but my guess is that you will have the same problem again and again until your xp dies.
If you can't afford both a new computer and updating to EQ7, then:
a) run the "defrag" program on your xp computer. It will remove old files and unnecessary .tmp files. That will help with the memory. The operant word is "help", not fix completely. Do this on a regular basis. (You can schedule it for times you are not usually on the pc.) Note that old .tmp files will slow your pc down significantly and eventually kill it completely. Unfortunately, I learned that the hard way - it happened to me!
b) To reduce competition for memory, schedule automatic updates for a time when you know you will not be using the computer.
c) Only use EQ5 after you have exited other applications like web browsers or word/excel on your pc. Again, that reduces the competition for memory.
d) When using EQ5 save your work frequently.
If/when you upgrade to Windows 8 and EQ7, this is still very important to do. Failures can happen anytime, no matter how new/old the pc is.
(We bought a Window 7 pc about a year ago, brought it home, did all the stuff you have to do with a new pc - and then it failed over and over until we decided it simply didn't work. We returned it for another and have had no trouble with it.)
e) Save a copy of ALL your important files on a thumb drive. Save files you are using to the thumb drive every time you update them. That way even if your pc fails, you have your work saved. I am a bit paranoid about this and save to two different thumb drives because one of my thumb drives failed.
I hope this helps you. Good luck.