Not much to go wrong during the install of RealVNC that would present this problem. This can happen even after VNC is working fine with windows authentication. One day it works but the next day you are prompted with an error message with no access check logs.   If you change the authentication option to RealVNC password it connects with no problems.  The problem is not with RealVNC, not exactly anyway. Your windows event logs are full and not being cleaned out. Chances are you have a app that is filling up your event log every few minutes.  If you can’t stop the errors from filling up your log, then increase the size and elect to have it over write when full.  Probably not a setting recommended by your system admin but chances are you never check your event logs anyway.  Clean out your event log and everything works again.
Event Logs can be accessed through the Control Panel.  Administrator Tools is the default location, look for Event Viewer.   Click here MS KB308427 for more details.

Check Environmental Variables…  This is most likely from a security program that cleans your tracks.  Something like CCleaner or Ultra Sentry just for example.   Most of the time your user “TEMP” folder is located in your home folder of document and settings, cmd line prompt and enter ”CD %TEMP%”.   Also you can do a cmd line and enter “SET”.  This will display the TEMP path along with other information.  This is not the only TMP folder that is used.   The tmp folder that is probably causing the problem is the System TEMP, “C:\Windows\Temp”.  Check to see if the folder exists, if not create the folder.  
Other things could be the enviroment path setting, these can be checked or added from the System properties.   Open the System icon in the Control Panel, click Advanced tab and then Enviroment Variables.  Make sure there is an entry for TEMP and TMP for both the user and system.  

When XP boots and a blue screen pops up with can not find AutoCheck or Ssiefr.e skipping autocheck then something has changed your BootExecute entry in the registry.  This is most likely from a Defrag program or Mal-ware scanner that does boot time checks.  It seems many un-installers leave this entry left for us to remove or has removed the whole entry period. 
This key here is the culprit.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager
“BootExecute”=”"

Now the normal entry should be this. 
“BootExecute”=”autocheck autochk *”   This is a multi-string value entry.
If the entry is missing, manually add it back.  This presents the missing AutoChk message.  When you add the line back, upon the next boot you get a prompt to run check disk… Then no more prompts afterwards.  Some of the others I have run across.
“Ssiefr.e” is from SpySweeper. 
“OODBS” is from O&O Defrag.
“UDBDef” is from UltimateDefrag
“pdboot.exe” is from PerfectDisk
If you still have them installed leave em alone, if not delete it.

Nero 7 installs some crap called Nero Scout.  If you installed Nero, upon reboot you will find a search bar within your TaskBar.   Nero does not include an option to disable this or not to install this pest.  So here is the quick way to disable this once installed.
Click Start Run and paste this whole entry and Enter.  Optional you can open a cmd line and run each regsvr32 entry.  The /u is uninstal and the /s is silent (no confirm or fail notice).

Cmd.exe /c regsvr32 /u /s "%ProgramFiles%\Common Files\Ahead\Lib\NeroSearch.dll" && regsvr32 /u /s "%ProgramFiles%\Common Files\Ahead\Lib\NeroSearchBar.dll" && regsvr32 /u /s "%ProgramFiles%\Common Files\Ahead\Lib\NeroSearchTray.dll"

Once completed you will see the search bar disappear…  Done.
NMIndexingService service can be stopped but will continue to start when needed, set at Manual.   Kill the startup entries for Nero and it will stop loading at boot.  NeroCheck is ok, this only checks to see if the aspi entries has been altered, then ends.