Boot into Safe Mode, run CHKDSK and defragment. Since you deleted a lot of files, the HD may be heavily fragmented and this may be a reason for the slowdown. With file sizes getting so large...

Did you do the usual..check for viruses, spyware? Scan disk?
Cleanup temp internet files and folders..also defrag.

Also check your HD for fragmentation. Run a defragger if you havent in a long time. May help speed up the system.

Defragmenting works best when there arent other processes that clash with it, hence the safe mode option or as CW said End Task option. A good automatic defragmenter however does this efficiently in...

Try running defrag in safe mode, and also check if there is enough free disk space for a proper defrag.

Go to Start>Accessories>System tools>Disk defragmenter>select the drive you want to check the frag levels for and click analyze. this will advise you on whether it neeeds a defrag or not. third party...

Have you checked your fragmentation levels lately? Maybe running a defrag would speed up the startup.

Program errors, failure to access data etc could be because of high fragmentation levels of your HD. You could run a defragmenter and see it it helps.