You may have too many things running on boot that is eating system resources.
You do not say which OS you are using, but if win98, then start --> run --> mcsonfig and choose selective startup. Then on the startup tab, remove everything except for systemtray and scanregistry. then Ok and let windows start. then test. everything you uncheck there in the startup tab takes resources from the system; many of the programs will still run... but they won't be pre-loaded into memory.
If you are using another OS, then I recommend that you look in the registry. Start--> run --> regedit --> HKEY_LOCAL_MACHINE --> Software --> Microsoft --> CurrentVersion --> Run (runonce, etc). Whatever is listed there in the Run sections run at startup and eat system recources. You can delete them, just make sure that you know which you are deleting (and re-install the program if it creates other problems). It would be a good idea to backup any registry keys before deleting them... just in caseyou need to recover.