does some viruses of xp effect the performance of ubuntu

Malware for Windows usually does not affect the performance of Ubuntu, but since Ubuntu is capable of running some Windows software with Wine (http://winehq.org), potentially malware could be running via Wine. There is also malware for Linux-based systems, though it is extremely rare compared to malware for Windows systems. Provided that the malware is not extremely sophisticated, if it's running via Wine then you should be able to disable it by uninstalling Wine (though you would also lose your ability to run Windows programs in Ubuntu). You can uninstall Wine in Applications > Ubuntu Software Center (the package is called wine), and see if the performance of your system improves. You can reinstall it later.

You may also want to see what programs are using a lot of system resources (System > Administration > System Monitor). Please note that your computer can be slowed down by overuse of resources *other* than CPU time, and also that highly sophisticated malware can hide its own resource consumption (see http://en.wikipedia.org/wiki/Rootkit).

As for running AntiVirus software on Ubuntu, your best bet is probably ClamAV, as it is available natively for Ubuntu and can be installed with the package manager (just install the package called "clamav", which you can do in the Ubuntu Software Center).

If you want to ensure that malware is not running, your best bet is to completely wipe all the disks on your computer and reinstall your operating systems. Assuming that you want to continue running Windows as well as Ubuntu, it is best to install Windows first (as installing Windows on a computer that currently runs Ubuntu would require that you manually restore the GRUB boot loader).