Enroute Controller described a loss of separation event during holding procedures when an aircraft entered the holding pattern at an occupied altitude shortly after hand off; listing only a lack of vigilance as a causal factor.

Narrative

I assumed my sector; which already had a D-Side staffed in the hand off position due to equipment issues. I was briefed that a major airport was shut off due to weather and we were already holding. The Supervisor made the outgoing R-Side relieve the existing D-Side to remain and help. We were told we would be holding for about an hour or so. I believe we had between 5 and 7 aircraft in holding along with some overflights. The numbers grew fairly quickly within what seemed like minutes. I accepted the closed airport arrival hand offs from the neighboring Center; along with multiple overflights from other sectors. Frequency congestion became a factor as aircraft began inquiring about the delay and alternates. My D-Side did a great job coordinating altitudes with adjacent sectors. As aircraft would check on; I would evaluate if they were a factor for my holding stacks. This seemed to be working. When an A320 checked on; it took a few more seconds than usual to locate him. When I did; he was already within the lateral confines of my holding stack at the intersection. As I noticed this; I quickly scanned for traffic. I concluded that he was okay. It seemed like only minutes went by when the controller on the adjacent sector commented; at the same time I saw it. Within seconds; and as I began turning both aircraft; the Conflict Alert went off. Not more than seconds after I issued vectors; both aircraft responded to TCAS. I stopped issuing any instructions after that and told both aircraft to let me know when they were done responding. After they were separated; the A320 was cleared direct to the VOR; and the MD80 was eventually put back in holding. I don't know if there is anything outside of my own self evaluation of how I scanned my overflights that could have helped. This was my error. I wasn't able to scan my overflights until they checked in. I should have seen; if some one's check in was delayed; so would their traffic search [be delayed]. I don't think there were any bad procedures; plans or configurations in place.