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What version of Parallells are you using?
What operating system are you wishing to install on it?
Can you give us a little more detail about at what point you are having issues?
So you have installed parallels and XP however when you try to run arallels it gives you the error, Is that correct?
If so can you confirm you are using this version of parallels http://www.parallels.com/en/download/desktop/
If you are could you try these troubleshooting steps http://kb.parallels.com/en/4790
You will not have to reinstall XP as the virtual disk should be unaffected.
Let us know what happens.