Here is an example of how you might accomplish this.
<chain>
.
<packagegroupref id="AccessDB" />
.
</chain>
<fragment>
<util:filesearch id="AccessDBInstalled" xmlns:util="#unknown">
Variable="AccessDBInstalled"
Path="directory\path\to\check"
Result="exists" />
<packagegroup id="AccessDB">
<exepackage id="AccessDB">
DisplayName="Microsoft Access"
Compressed="yes"
Cache="no"
PerMachine="yes"
Permanent="yes"
Protocol="none"
Vital="yes"
SourceFile="path\to\accessdatabaseengine.exe"
InstallCommand="/quiet /norestart"
DetectCondition="AccessDBInstalled"
InstallCondition="(VersionNT >= v5.1 OR VersionNT64 >= v6.0) AND (NOT AccessDBInstalled)" />
</exepackage></packagegroup>
</util:filesearch></fragment>