Do you use UBCD4Win? UBCD is another project.
Read the integrated help files, create a own plugin.
What's your file name?
Example Scan.exe:
Create a file plugin\AntiVirus\scan\scan.inf.
Copy file scan.exe to directory plugin\AntiVirus\scan\files\.
Add scan.exe to RunOnceEx.
CODE
; PE Builder v3 plug-in INF file
[Version]
Signature= "$Windows NT$"
[PEBuilder]
Name="Antivirus: Scan.exe"
Enable=1
[SourceDisksFiles]
files\scan.exe=2,,1
[Software.AddReg]
0x1,"Microsoft\Windows\CurrentVersion\RunOnceEx\900Scan","150","||RunHide scan.exe"