You can't uninstall it, You can format the partition it's on, but this can have unwanted results.
If you installed W7 while you had XP on another drive or partition, then the bootloader will be on the XP partition. To get rid of it would be a fairly convoluted process and maybe more trouble then it's worth unless you're prepaired to go through the process.
If it's on a seperate drive it's 4 pages of text to read and follow.
If it's on the same drive on another partition, easiest thing to do would be to start again.
Assuming you are using the upgrade version of W7 and can still boot into XP.
Start the pc and log into XP format the W7 partition. Restart with the W7 disk in drive and boot into W7 and use the custom install option, select the XP partition to install W7 on, format it, or even delete both partitions and make one new one using the whole drive (your choice)
Compliance checking is done during the bootup process, so it's ok to format XP from W7.