I have run Windows XP on a 486 133MHz system before, mostly just to see if I could. The computer had 64MB of RAM and while it was far too slow for my preferences, it was completely usable for simple web browsing and word processing (PowerPoint presentations would probably be OK if you don't use animated transitions).
Before the system died, I have also played DiVX and DVD movies in XP on a PII-333 laptop with 128MB of RAM, so PowerPoint should run reasonably well on your PII-300 system, particularly if you can add some more memory to it. Again, animated transitions could cause problems but basic presentations should be OK. Web browsing was perfectly acceptable, and despite what Green Man said, it only took about 5-10 seconds to open Internet Explorer, which may be "too long" if you're used to a modern machine, but it's totally usable.