The error "stmgr has caused in error in WINTRUST.DLL" is relatively
unusual however a fix is known for this problem but this isn't going to
fix your other problems such as the loss of scandisk and defrag and the
inability to run any program other than Internet Explorer.  Even if I were
to lead you through the quite lengthy procedure to fix the stmgr/wintrust
error there is no guarantee that you would have any usable system restore
checkpoints or that one of these would restore the missing files.

What worries me somewhat is your statement that you are not sure of what
took place as having some idea of the origin of the problem might help
establish whether it is worth the time and efforts in repairing the system
or whether it might be better to bite the bullet and reinstall the
operating system and restore it to the state it was in when the machine
was first purchased.

Regardless of how you/we proceed I would suggest that you start to try and
move any valuable data such as documents, spreadsheets, photographs,
music, e-mail messages, saved passwords, etc. to backup or another PC.