"The operation cannot be completed because the original item for "Shares" cannot be found."
Searches on Google came up with plenty of solutions for this error, but none of which worked in my case. So in case it helps somebody in the future, here's how I resolved it.
A little investigation revealed that I could share other folders and access them fine from the MBP, just not the external drive. I eventually figured out that I just couldn't share things that were large. Since my external drive was 500 GB, it easily fell into this category.
This size restraint on sharing was fixed by following instructions here:
Basically, there's some parameter (on the WinXP machine) you have to adjust to allow larger things to be shared. The parameter is adjusted/added using regedit. I changed it to the max value of 0xC (12), and that worked fine.