Gets really sticky depending on why you say you are modifying things.
For example, the DMCA protects you if the modification is for educational purposes.
If they want to go down the route of arguing a contract violation, then the question about whether or not the contract is fair comes into play. You can't write a contract that takes everything and gives nothing in return. So these license agreements which basically say "You can't don't own anything about this software, we have every right to sue the pants off you if you upset us, and we accept no liability if our software does anything bad" may very well be unenforceable (Doesn't come up that often because people are relatively sane).
The other interesting thing here is that these licenses may be doubly unenforceable as companies have been adding the "you accept all terms and conditions if you push this button". They are right in the same line as the "Warranty void if you remove this sticker" notices.
For example, the DMCA protects you if the modification is for educational purposes.
If they want to go down the route of arguing a contract violation, then the question about whether or not the contract is fair comes into play. You can't write a contract that takes everything and gives nothing in return. So these license agreements which basically say "You can't don't own anything about this software, we have every right to sue the pants off you if you upset us, and we accept no liability if our software does anything bad" may very well be unenforceable (Doesn't come up that often because people are relatively sane).
The other interesting thing here is that these licenses may be doubly unenforceable as companies have been adding the "you accept all terms and conditions if you push this button". They are right in the same line as the "Warranty void if you remove this sticker" notices.