Nothing prevents you from installing 32bits Windows on a 64bits CPU, my Athlon 64 laptop has 32bits Windows on it. This does limit you to ~3.5GB of max usable RAM and locks you out of 64bits instructions. Doesn't make much sense to do so on a CPU from 2011. If you have more than 4GB installed and really have 32bits Windows installed, you may really want to re-install with a 64bits version. If you have a retail version, you should be able to use the same product key regardless of 'bitness'.
Not all Xeons are 64bits though: Intel has been using that brand since the Pentium II days and x86-64 wasn't a thing yet back then.