Files converted to txt files

Tech-Archive recommends: Fix windows errors by optimizing your registry

I have a 5 year old Sony desktop running Windows XP. The computer has a
partitioned hard drive and the C drive filled up to near capacity. I have
been trying to take off or move programs, but none of the utilities work
except Windows Explorer. When I look for files such as cleanmgr they are in
their correct directories but appear as txt files.

I thought originally the files would not work because of lack of processing
capacity, but now I am wondering if this is a virus. Can anyone help?
.