Statistically speaking, starting up from cold is the time when your Mac is at the greatest risk of suffering a problem.
Inmany cases, it will be software rather than hardware that causes anystrange behaviour, which is the lesser evil since it's usually easierto fix.
Failure to boot up reliably can be symptomatic ofsomething more serious, but much more common problems are slow boottimes and slow logins. Let's look at these in turn.
Starting up
YourMac loads an awful lot of code before it gets to the Finder view you'reused to seeing, and the longer you've had the Mac, the greater thechances that there's a lot of legacy stuff hanging around which canslow down the startup process.
A "vanilla" or completely freshsystem will generally start up quickly on any modern Mac, but as youinstall programs over the years and fill up your boot drive, thecomputer has to perform more disk activity and sift through morestartup items, even unused ones, before it can finish booting.
Oneremedy for this is to use an uninstaller program like AppZapper orAppDelete to delete any unused apps on your system, including allassociated support and library files. You can trash the application'sfolder, but this by itself won't get rid of other resources placedelsewhere in the system when it was installed.
If you go toMacintosh HD > Library > StartupItems you will see theuser-installed things that your Mac is loading as it boots. Typically,these are services that applications or peripherals need to runproperly such as printers, scanners or dongle security enablers.
It'spossible to delete items from this folder, but be careful that youdon't trash anything that you actually need. If you are absolutely surethat you can go ahead – for example, if you find resource files for aprinter that you no longer use – removing them can help improve boottimes.
This folder is part of the system and is read beforeanyone logs in, so changing its contents will affect all user accounts.In addition, keeping your boot drive relatively clear by offloadingmedia files and large iTunes libraries onto secondary drives will helpoverall performance.
Slow logins are another potential problemand are solved in a similar way to that just mentioned. In this casethe offending items are likely to be located in the System Preferences> Accounts > Login Items section.
When you install someperipherals like scanners or printers, or if you ever choose to allowan application to open at login, these things will appear in this list.Each one must be loaded by the system, which takes time.
If you select any unwanted items and hit the minus button to remove them you should notice an improvement in login speed.
Nuts and bolts
Ifyour Mac exhibits startup problems following a system update, you mayhave to boot from the OS X install DVD and run disk repair on the bootdrive, then try a restart.
If you are running OS X 10.5 andregularly back up using Time Machine, it should also be possible toroll back the system to its pre-updated state by booting from a systemdisc, connecting the backup drive and choosing Restore System from Time Machine. This will replace the corrupted system with a working one, though it's something of a last resort.