My name is Vishal and I work for the Social Media and Community Team at Dell.
As you have Dimension 3100 which is very OLD computer, hence I will recommend you few below steps in order to reduce the boot time:-
1) Uninstall all unwanted program which you dont use.
2) Increase the memory (RAM) of your computer.
3) Run Disk CleanUp which you will find under start >> All Program >> Accessories >> System Tools, and run the full clean up which will help you delete temp files.
4) In case this still doesn't help, I will recommend you clear Operating System Re-installation. Before you do OSRI please back all your files folder and everything which is important for you.