Just guessing here.
1. Maybe your profile's got a huge number of programs that load in the background. Remove programs that are not necessary to your daily PC workload.
2. You may have installed/uninstalled alot of programs and registry is kinda full of unused files that are still loaded or even files that are needed but can't be loaded. Just run a registry cleaner.
3. Hard disk free space is at critical level. Just delete porn movies

4. There's a weak sector where a program loaded at startup is residing ... just run checkdisk.
5. Could be fragmented hard disk. Just defragment
6. I dunno if there's a new virus that affects profiles ... probably loads itself up and tries to propagate during startup
Boot using a clean bootdisk and scan for viruses.
Can't think of anything else as of the moment.