If you are using windows XP. Then you could try these;
Start > run > msconfig > start up tab. Untick every thing apart from your antivirus and firewall, if they are listed. It will require a reboot.
You could check the size of your paging file ( virtual memory ). This is in control panel > system > advanced > performance > advanced > virtual memory. What readings do you have there? This can be tweaked to get better performance, but the size of your hard drive / drives, available space on them and if you have any partitions would need to be known to get the best performance.
You could then check on windows task manager ( ctrl + alt + delete )/ processes/ memory usage, to see if anything else is hogging the memory.