So as the title describes, Excel is running an entire core at max, and it makes the file nearly impossible to work on.

I'm working on a file that has this issue.

Under 2003, there is in fact one sheet in this workbook that can cause this issue, but only when that sheet is active.

In 2010, whether in Compatibility Mode or after being converted to an xlsx using the built-in tools, at all times while the file is open the spreadsheet uses 100% of the CPU, Except: when windows like the Options menu or Save As... Menu are open. 

Excel 2010's Workbook Calculation is set to Manual.
There is no VBA and no macros in the file.
There are no apparent links.

Any ideas as to why this file is doing this? Why is there a difference in how 2003 handles it when compared to 2010? I'm pulling my hair out a bit on this.

Hi

Follow these steps to troubleshoot the issue:

Create a new workbook in Excel 2003, select all contents from the original sheet that cause the issue, copy it to the new book and check whether the problem still occurs.

If the CPU still hit 100%, there may be something corrupted with your content in the old sheet, we can create another workbook and copy half content of the original sheet into it, see if it works well.

If the issue comes again, replace the copy with the rest contents then check.

After repeating the action multiple times, we can locate where the problem actually exists in your original sheet. Then modify to get it working well.

As Excel 2003 file format is binary-based, while Excel 2010 file format is xml-based, opening a binary-based file in Excel 2010 may consume more CPU resource.

So there is a difference in how 2003 handles it when compared to 2010

Regards