You can run UltraSentry either with an account which belongs to the administrator group or on Windows 8 / 7 / Vista you start UltraSentry with right clicking on the shortcut and choose "Run As Administrator".
ATTENTION: Don't use Clean File Slack Space on the system drive. The system drive is the drive which contains the Windows directory. That can harm your Windows installation.
On system drive you should use first Windows disk cleanup tool and delete among the temporary files also the shadow copies and older system restore points, next use the Windows defragmentation tool, next use again Windows disk cleanup tool to delete shadow copies created on defragmentation (only Windows 8 / 7 / Vista) and finally use UltraSentry to Wipe Unused Disk Space (not available in US v13.00) which can be safely used also on system drive.
The problem with Clean File Slack Space on system drive is that UltraSentry must increase every file on disk so that all clusters used by the file are completely occupied by the file. Then US can make the secure deletions of the bytes in the slack space of the files and finally reduce the file size to original file. On system drive increasing the file size and cleaning the slack space works on every file, but decreasing the file size back to original size fails on files which are special protected by Windows. As the file size changes on those special protected system files, the Windows File Protection (WFP) starts restoring the system files while US is still running. On my Windows XP computer it asked for the Windows CD-ROM from which I installed Windows on every modified system file which I cancelled the first 40 times. Then WFP becomes so annoying that I put the Windows CD-ROM into the CD drive. That was not a good idea as Windows now replaced some newer system files updated with Windows security updates by their older versions from CD. The result was that some features did not work anymore because of the mixture of older and newer system files. For example access to WWW and the defragmentation tool did not work anymore. I needed to restore my system drive from an image backup to get Windows fully working again.
I reported this problem with file slack space cleaning to IDM some months ago for US v22.214.171.1241 and suggested to make this feature not available for the system drive for safety. I have not yet installed US version 13 and therefore don't know if IDM followed my advice or skips now the protected system files on cleaning file slack space.
Please note that not US harms the system files. The problem is WFP which allows increasing (modification) of the special protected system files, but prevents decreasing them back to original size and later replaces the modified special protected system files (bytes with hexadecimal value FF appended) by older versions not being fully compatible with other, newer system files.