Rundll32.exe is a Windows file. It is used to run code from DLL files without having to use a specific executable. It is needed by the OS, so don't try deleting it! Let's use a great tool to see what DLL it is running:
Download Process Explorer from Sysinternals, a tool that shows a list of processes and allows to see much more info than Task Manager. Run it and you'll see a list of running processes. You may see that there are multiple entries for rundll32.exe, don't worry. Find the right one by checking the CPU column for each of them and right click the one with the high CPU usage. Click Properties and a new window will open. Click the Image tab and check the text in the Command line box. You'll see something in the style of "C:\Windows\SysWOW64\rundll32.exe" C:\MyFolder\MyDLLName.dll,MyFunctionName.
"C:\MyFolder\MyDLLName.dll" is the path to the DLL that rundll32.exe is running. That will let you know what's hogging your CPU. When you know what software the DLL belongs to, you can decide between keeping it or removing it. If you want to keep it, look for updated versions of it, as it may be a known problem.
Since I don't have that information I can't tell you anything more on how to solve it. Please post the text from the Command line box and I'll expand on this answer.