Not surprised your PC is struggling. They are pretty beefy progs. I would suggest you need:
New processor - I think the Athlon XP 2400 + would be sufficient
Min 512Mb PC 2700 memory
Big HDD - say 80Gb minimum
New Graphics card - say Raseon 9200 with 128Mb DDR RAM
It may be that your old case will not cope with the cooling requirements of all this new kit. Also likely that the PSU will not cope.
So ..... really would be better to buy a new PC and network the old one as online backup and storage. If you have no need to play games, you do not need to spend a fortune on a fast graphics setup, the card mentioned above should more than cope with anything autocad and photoshop throw at it. Watford have some pretty good basic machines at reasonable prices (around £500) with built in modem and built in LAN and on board sound, and while you only have limited configurability on their web site, they will alter them around to your hearts content if you speak to them on the phone. click here