Something unusual is happening

I run a dual boot XP and Windows 7 system. I use AVG antivirus 2012 with PCTuneup, Peerblocker, SpyBot, Tea Timer, and Malware Bytes. With in the last month my mbr on the XP partition has increased to around 60 ~ 80 GB in size as reported in PCTuneup. There is no detectable cause and no evidence of malfunction. Running a defrag on the mbr shows a normal size record. I suspect something has occupied this portion of my hard drive and marked those clusters as mbr when they are not.

PCTuneup reports approximately 1000000+ sectors of mbr (43 blocks) but registry analyzer shows 75mb for my XP partition. Windows 7 shows 4 blocks totaling 450.5 mb in PCTuneup. My hard drive is 1 Tb partitioned into 2 halfs XP about 380gb and Win7 about 480gb. Just wondered if anybody has seen this before