Look at START RUN msconfig.exe STARTUP to see what you have starting up and reduce them to as few as possible.
Suggest you try this site to determine what services you really need - select the OS version and Service Pack that applies to your PC:http://www.blackviper.com/
Make sure you follow the advice to use START RUN Services.msc to change any services.
Depending on what programs you are running you may need more than 1GB RAM.
Run Disk Cleanup and remove old System Restore points which take up a lot of space and slow the PC down:
To clear old System Restore sets - Log in as Administrator, Open COMPUTER, Right click on C: Drive, Select PROPERTIES, Hit DISK CLEANUP and let it do its work. When the results come back Select the MORE OPTIONS tab at the top and then hit CLEAN UP under the System Restore and Shadow Copies heading. Then keep selecting Delete and OK until it has cleaned everything up. You should then Defragment your C: Drive.